Foresight: Contender for Best Spell?
I recently played a 20th level wizard in a one-shot and was stressing over my 9th level spell choice. Wish and simulacrum were banned, so that combination was out. I reviewed Treantmonk's most powerful spell series, but I didn't want to play the one-shot shape-changed, so shape change and true polymorph were out. Additionally, I knew we would be fighting dragons, and prismatic wall doesn't protect you from breath weapons. Eventually, after suffering from analysis paralysis, I settled on foresight for the sake of simplicity.
The traditional criticism of foresight is that it's mostly a defensive buff for wizards since they don't make attack rolls. But it has been quietly buffed in 5.5. I should have prepared chromatic orb, as I would have been almost as effective as a sorcerer with it (no ability to empower it, but that's not too important once you are certain it will bounce). Additionally, I was able to use sylune's viper on the boss and hit it every time. I was holding wall of force on a non-boss enemy, so I didn't get to try this, but there is synergy with summon undead. The putrid form can paralyze enemies that are poisoned. None of this requires a saving throw, only that you hit and the putrid form hits once. So the tactics for using foresight as your ninth level spell would be to throw down your control spell, then use chromatic orb if you are fighting hordes or sylune's viper in a big boss battle.
I realize there are condition and damage immunities to deal with, and if you know you are fighting undead, for example, this combination will not be very powerful. But as a high level wizard you have room for keen mind and expertise in investigation and arcana, so you can bonus action study enemies to learn about their immunities.
My title is mostly thought-provoking, as the most powerful spell depends on context, what is allowed at your table, and if you can look yourself in the mirror if you shape change into a genie multiple times to get multiple wishes. Wish and shape change will always offer versatility and power but require you to do a lot of homework.
In a single battle, foresight probably does not win out, especially if you are fighting enemies with poison and elemental immunity. However, the battle field might not set itself up for an immovable prismatic wall, and even if it does, that's one encounter. If you are doing a 20th level dungeon crawl, foresight is very strong and will help you through multiple encounters. It buffs your saving throws, skills, and defenses. It allows you to adapt your resource usage to the challenge at hand by using low level spells like chromatic orb and sylune's viper very effectively. This preserves resources for a long adventuring day. Anyway, this is food for thought rather than a strong opinion of mine. At the very least, foresight is underrated now that wizards have some great options that require attack rolls.