Since I am (still) a fan of Diablo games and had a lot of time on my hands I went a bit crazy and rerolled multiple times, here is my personal opinion on most ^([Edit goblin here]) all of the classes we have in the game atm:

Warlock (stopped on T11):

Pro:

• lots of builds
• new and amazing visuals
• some funny stuff you can do and lots of great defensive tools at your leisure
• Can be melee, can be range, can nuke screens, can be an assassin, can basically do the exploding chicken build from D3 Witch Doctor

Con:

• a bit clunky at times and you will not be able to see anything on most builds due to the visual clutter that are most of warlock's sfx
• some builds are just a bit broken (Apocalypse hitting in the hundreds of trillions is a tad silly imo)
• while you have great defenses, surviving in t11+ will take some investment because you will probably be blind to any ground effects and need to adjust your build

Barbarian (stopped on T8):

Pro:

• op af
• lots of weapon slots for mythics and other fun aspects
• one of the most relaxed ways of farming in the game with WW barb
• Ragdolling stuff with big hammer smash is still as fun as on day one

Con:

• still mostly exact same builds
• still one to two skills plus all shouts gameplay
• a bit...basic ?
• Another class that scales a bit too well imo (trillions just seems silly no matter what, then again at this point almost every class has figured out a way to see those numbers pop up so I guess it's more a general issue than specific to Barb or Sorc)

Druid (stopped on t12+)

Pro:

• best skill tree of all classes imo
• tons of fun build ideas to try
• omnistorm + wolf set charms is basically flickerstrike in a roundabout way (albeit you don't hold down right click but if you bind it to mousewheel it's kinda the same thing) and totally insane
• Companion druid is another super chill way to just blast t12+

Con:

• still some clunkiness at times with teleporting into the ground or getting stuck somehow (esp. with Wolf set teleports)
• transmogs are pointless because you are either a bear or a wolf now for 90% of all builds you will see
• Wolf and Lightning set are fun, the others are kinda boring imo
• one major defensive aspect I wanted to build around is broken (the resolve stacking one) and that put me off trying to keep improving my gear [Edit] : I'm talking about Glynn's Anvil aspect that should give dmg reduction based on resolve stacks. This currently doesn't work at all so it's kinda pointless which is sad because this aspect alone would probably solve most survivability issues on almost any build because you can temper resolve stacks onto armor which should net a ton of dmg reduction from this aspect alone)

Paladin (stopped on t11):

Pro:

• cool class
• great Transmogs (T2 WoW does look nice even if expensive af) -- but you wont see tmogs because 90% of all builds want permanent arbiter form
• Being able to support teammates with auras feels great and they also enable Paladins to get some free crit / res very early on
• iconic items like Herald of Zakarum and the sound of hammers being summoned is triggering my D2 nostalgia in all the best ways

Con:

• auradin is kinda "eh" now
• surprisingly squishy considering we have items like Zakarum and Auras
• you constantly have to stack arbiter form stacks to actually do dmg which gets very tiresome imo
• it feels like there aren't that many interesting builds because it's either Arbiter stacking + dmg skill or "the other one" (Thorns Captain America)

Necromancer (stopped on t12+):

Pro:

• Blood wave is still fun and reigns supreme
• Mostly there build wise overall imo and now with unlimited mobility and teleports galore
• Lots of fun uniques or runewords that automate some of the more annoying parts like cursing or Corpse Tendrils

Con:

• Minions still have subpar Ai imo
• Identity wise it feels more and more like "sorc but sometimes with skeletons"
• Sometimes you forget that some skills need corpses to work and will feel silly
• Can we please turn off the shouting from ultimates ? Blood wave Necro is basically "HYAAAAAH HYAAAAAH HYAAAAH" until your ears bleed (guess on theme there but please Blizzard my ears)
• just doesn't quite..."click" for me personally

Sorcerer / Sorceress (stopped on t12+++)

Pro:

• completely and utterly broken dps
• insane mobility to a degree that playing it almost becomes a hassle because you can barely keep up with the speed
• fire shield makes Mephisto fight almost bearable

Con:

• too easy and too strong as Ball Lightning sorc (you basically get all the needed uniques and the set, casually stroll into t12 as a god and only go up from there)
• it seems like build wise it's still lightning sorc and lightning sorc and lightning sorc and maaaaaaaaaaybe something - NOPE it's lightning sorc with lightning blizzard now

[EDIT ME HERE AGAIN - due to popular demand I have now also gotten a rogue and a spiritborn to max (thanks for whoever pointed out to me that you can just use max lvl caches on new classes to instaboost them) and put them through some speedgearing process to just get a feel for them so here goes for all you dex class enjoyers and thanks (no thanks 😛) for making me do this xD -- however enjoy these 2 opinions with a larger grain of salt than my entire ramblings beforehand because I really didn't spend much time on them, mostly I just did it because I felt bad about leaving all you rogue / spiritborn enjoyers hanging]

Rogue (stopped on t8ish due to time constraints)

Pro:

• leveling is fun and fast and we still are one of the more mobile zoom zoom classes
• I felt more tanky than for example Paladin probably because I mostly either spent time being in stealth (the new stealth set charms are amazing) or shot from a distance / or just froze everything and then blew it up
• Tons of build variety (honestly I was positively surprised here) - you can go melee, freeze, poison, shoot and stab things into the ground in a variety of ways - pick your poison (in a good way) [sry for this lame pun]
• Freezing mobs and then watching them explode just feels (insert *noice* meme dude here)

Con:

• The stealth set charms are amazing, anything other than those feels kinda "eh" mostly because stealth set just reigns so hilariously supreme
• Lots of builds have a bit more positioning gameplay than I generally like until you really get better gear (which I didn't quite reach in the short time I took to just speed blast level it to appease the rogue enjoyers reading this post xD)
• Somehow they have become the "here comes the next bugged build clearing pit 150 in 30 seconds" class so I dunno why but it seems like rogue has the most wonky interactions
• the new / old "GoD" whirlwind rogue just feels un-rogue-ish to me, dunno why but I feel like after so many years of doing that build in diablo 3 I'm kinda over it at this point and not every class needs to be whirlwind barbarian

Spiritborn (stopped on t6 because I kinda ran out of easy gearing stuff at this point)

Pro:

• Gorilla big hand smooshing mobs into paste has always looked fun, is fun and will probably forever remain fun
• Poison creeper builds are finally viable from what I could tell in my arguably way too short time trying it out - YAY 😃 If you like summoning a big centipede and poisoning your enemies you're definitely going to have a good time
• If you liked perma evade gameplay we found a way to replicate it once again now with a bit less clunky mechanics (no need to bind evade on your mousewheel and have an unlockable mousewheel to truly zoom)
• Generally a mobile class that "feel" more dextrous than others somehow (can't quite put a name to it)

Con:

• It's just...I dunno - the identity of Spiritborn seems weird to me. Am I a rogue who got comfy with some spirit animals ? Am I a monk that gave up fists for more stabby implements ? Am I some kind of animal summoner ? Especially now that we're out of Nahantu I'm having a hard time connecting with this class
• The poison set is super strong but also has this weird thing of having to kill something non boss for it to trigger which seems a bit convoluted to me
• Felt rather squishy on anything other than gorilla (mostly because gorilla just deals its damage faster than stacking poison dots from what I could tell) even though we have great mobility and some ok defensive tools

Now I am sitting here on paragon 236 after a week of insane grind and am wondering what to play to finish the full season journey with and just goof around in t12 (no desire to push much really). Anyway maybe this helps someone, if you have any questions about builds used etc. just comment something --- If you read this whole thing I want to first congratulate on hitting 40 years old (jk) and thanks for taking time out of your likely busy day to read the ramblings of an old ADHD addled gamer.

Cheers

EDIT: I am so super stoked and eternally grateful that this random post by silly old me blew up and made it to the top of reddit and I am grateful for every single one of you who took their time, read my post and commented on it - no matter what it was. To quote Keeanu: You're breathtaking !! I have gleefully spent the past 3+ hours trying to answer all your great questions and amazing feedback - however I do still want to get back to playing the game for a bit longer myself (granted a lot less than this past week because I do actually have a real life and I do enjoy touching grass now and then) so I will tone down the answering because most of your (great and awesome) questions have been answered in one form or another at this point either by me or some other helpful individual engaging with this post 😃

Still here is a general tip thing summarised from what I've answered most:

1. Get the warpath unlocks for lair boss nemesis+ portals to sometimes luck into insane loot drops from multiple bosses (just make sure you can survive something like Belial and Andariel stacking their mechanics)
2. Make sure you get the Idols of War "Ancestral Tribute of Armaments" unlock from the Undercity tree as soon as possible because you can 5 to 1 cube those tributes into a mythic tribute which you can then run in a stack for tons of mythics for very little investment -- one caveat is that you also really want the left side unlocks from that tree called "Trials and Tributes" for the portal pranksters that all but make multiple mythic drops a guaranteed thing on your run if you are the person opening it and take your time to full clear the undercity to get more portal prankster kills (so either have a lot of gold to respec or grind to lvl 6 which takes quite some time) -- however if you prefer playing solo the middle path is perfect for just getting more mythic drop chances so I really suggest going for that
3. Unless you really enjoy infernal hordes: avoid them - they take very long and are simply not worth doing beyond the seasonal unlocks atm
4. You can gamble obol items for easy blue / white ancestral items that you can then cube into random ancestral unique items with the cube recipe - it needs a somewhat rare material but if you play long enough you will accrue at least a couple of those mats quite easily which can help you
5. Belial is amazing. If you start doing tons of lair bosses (make sure to have a least the Two by Two unlock - even better if you get all the way to Ultimate Nemesis) you will get Belial husks which are the best way of avoiding the more cumbersome bosses (like butcher being super tanky or Astaroth and Bartuc being locked behind long grinds) to target farm certain uniques that you might want
6. If you can't seem to get certain aspects to drop, remember that you can upgrade Rare items in the cube to legendary while also using Cube mats to somewhat force a certain kind of aspect - a quick and easy way to maybe luck into that rare aspect that seems to be elusive
7. If you want to level fast and have a class that has some really strong screen clear ultimate skill, reroll your early war paths into undercity and try to craft / get some exp undercity tributes so you can double dip while also speed clearing because Undercity resets your ultimate all the time with its buff you get from doing the content
8. Another great tip for leveling if you want to do a new character is to just craft blue items at lvl 1, cube upgrade them with random affixes so you can temper your max level tempers onto them and roll into lvl 1 penitent with thousands of hp and aspected and tempered offensive gear to completely smash penitent
9. Almost all classes / specs are t10+ viable atm. Feel free to ignore the meta and just cook something up yourself, with sets and mythics being a lot easier to acquire than you might think (see points 1 and 2 about enhancing your loot) you can take any homegrown (and you're awesome for doing that !!) build into t10 at the very least. Don't be like me and just fotm hop around classes to find what you're enjoying, if you like something then do it and you will probably enjoy it a lot more than being the next Ball Lightning sorc that just melts t12.

Final Edit with proof of my lunacy