The Skeleton Archers in the Castle level of Blockland Main Adventure Mode are so loving hard. Took me like 10 tries to get past that level.It was worth it for Rotondo's Flaming Sword though. That sword was the only reason I could manage to get through the Airship level.
hotline miami and hotline miami 2 has the best AI i have ever seen
It's either sniping you from a window off screen or hearing you shoot that window that is 4 rooms away that makes them the hardestBut they spazz around the doors and don't notice the very brutally mauled bodies
Metal Gear Solid has some good AI.
Best and worst has to be stalker, hands down it can be smart enough to snipe you through a fog infested forest on the dark side of my starfish and dumb enough to get stuck on a can then spazz out and die