I've researched the stat transformation from main series games to PoGO. Mega Zygarde is some of the best realistic stat distribution possible.
I tried to create a minmax distribution for different BSTs (Base Stat Totals) with the most stat product (combination of all stats) possible. These custom Pokémon all lean to have a lot of HP and defence while having low attack. This is caused by the nerf given to mons with CP over 4000 at lvl 40 (attack increases CP at a higher rate). For a more attacker-focused spread, I simply maxed out one of the attack stats at 255. That left me with balancing only speed and bulk stats. What's worth mentioning is that these stats are artificial looking. Any similar distribution is highly unlikely to appear in real games due to extreme min-maxing and catering to PoGO calculations. That's why I think the best one will be something like Mega Zygarde. While it has high HP and attack, there are still some other stats around unlike in the theoretical that I found. At the same time, it doesn't waste too much on futile stats like some fast legendaries (e.g. Zacian Crowned, Shadow Rider Calyrex) or mixed attackers (e.g. Mega Rayquaza, Ultra Necrozma).
With all of that out of the way, here are the most optimal stat distributions for the BSTs of 680 (from box art Legendaries), 754 (from Ultra Necrozma), 778 (from Mega Zygarde). For the theoretically best box art legendary, the stats are 196/255/215/1/1/12 (HP/Atk/Def/SpA/SpD/Spe). The other two would have 201/255/231/1/1/65 and 217/255/239/1/1/65 respectfully. These translate to 356/215/358 (Atk/ Def/HP) for 680 BST (a beast far surpassing Zacian Crowned), 399/259/366 for 754 BST, and 399/268/391 for 778 BST. The stats in PoGO are nerfed by ×0.91 that always applies to non-mega mons which these aren't they are imaginary.
After this fine-tuning, I have a few key findings.
Firstly, you really want to have as many of your stat points as a single one of the attack and defence stats. The reason is that the higher stat of both gets a higher coefficient in multiplication, thus increasing the final stat significantly more. This is somewhat similar to the goals in main series games. Although, sometimes you can benefit from mixed attackers in that meta and having both defences high is good against different opponents.
Secondly, speed is mostly not worth investing in unless you attack and defence higher than ~200 both. Since Pokémon Go doesn't have a speed stat at all, it acts as a small multiplier for attack and defence. The points often would be better off somewhere else.
Lastly, due to the nature of stat product multiplication, the highest comes from equal stat spread. However, because of the difference in conversion formulas, defence is the least favourable out of the 3 main stats.
Hopefully, someone will find it as interesting as I did. Leave your thoughts in the comments.