Of all the video games you've played, which one do you feel had the best AI for the bad guys?

MGSV, I guess. I didn't play it enough but there seemed pretty capable.

Of the games I've played, I'd have to say F.E.A.R. for sure. The AI is both aggressive and smart. The game wouldn't feel the same with a less aggressive and smart AI.

I also liked the AI in the first Crysis. I honestly have no idea why a lot of people seemed to dislike it, because I thought it acted in a pretty smart and believable way. It also supported the suit powers very well. The AI may not have been very good at combat tactics but other than that, I think it was pretty good.

I'm really inclined to say the AI in Metal Gear Solid 2 and 3. They acted and behaved like normal humans with specific patrolling schemes; they were clever enough to respond to certain situations doing a specific course of action (that mostly gave you the disadvantage or got yourself killed), they were always curious about something weird on the enviroment (using a wetbox to hide indoors already give all the suspicion in the world), they responded to player imput, whether sound, a sneeze or irregular footsteps, command would call from time to time and if no one responded a team would be dispatched to check upon the situation; you could hold-up enemies and interrogate them, and they would mostly behave differently (there were the cowards who would just beg for their lives, or hardened soldiers that wouldn't scare until you start torturing them), and they had even secret, normal dialogue between them from time to time. In fact, the least human-ish thing about these were their patrol itself, which was a very game-ish route (from A to B, rinse and repeat), but getting spotted would made hell break loose, calling command and an armored team would then confront you, while other soldiers went to get some cover and support them. They'd use grenades if you made a fort in one room, and avoid/kick you if you get too close, followed with a point-blank shot.

Honestly, it was a truly advanced AI, one that rivals games from these years. On several aspects it felt like the AI from MGS4 and 5 took a massive step-down in some of the most simple aspects (Getting spotted in MGSV inmediately makes everyone aware of you and full alarms, whereas getting spotted in MGS2/3 by a single soldier would raise an alarm until you take that specific soldier out, or he calls for command and gives the full alarm). It's crazy how evolutionary most games have got, yet a sofisticated AI seems always problematic. But for some reason, MGS2 and 3 really nailed it with the AI, and while it can certainly be exploited (like every AI, let's be honest here, if you were trying to pull that monkey video on me for the comment), I can honestly point at it as some of this industry's finest enemy AI.

Honestly, I thought the AI in RAGE was pretty good.  They used cover well, could get around obstacles, and the melee runners were an absolute PAIN.  They would also use chains and vines to swing toward you, though that was probably scripted.

This. Also I'll fully admit there have been times in the first and second game where the AI has actually tried to hunt me down, times where I didn't hear a damn sound and then open a door only to find a squad of soldiers waiting there for me as if they knew where I was hiding. That shit was proper scary despite me having time slow powers. It's also quite refreshing because most AI these days don't bother searching areas to look for you or at elast a large area, most just stick behind cover or a wall and shout the same lines over and over like machines.

Rage i'd say comes in as another one with sort of decent AI along with the latest DOOM where I had a Hell Knight in the molten works map trying to track me down all the way from the highest spot. I ran in circles and went all the way to the top and he ended up tracing my steps back up to the top, all while looking for me in the vacinity.

It's a shame that most AI these days revolve around hiding behind something or repeating the same action over and over until a new set of results play out. Most AI in Fallout/ES games feel so hollow and droll to be around, makes it easy at times to see how they would react to moving or killing something in the space they reside in. I'm not asking for a Skynet level of AI, I just want one that knows it;s surroundings and doesn't revolve around kill or be neutral/friend.

To my knowledge MGS series are good.

Anyone who has played the original Perfect Dark on N64 will knows how good the AI is even compared to todays standard. The enermies can do so much. Still one if the best AIs iv seen in a FPS game. 

Honorable mention to Fear 1. They had awesome enermy tactics.

Depends on the genre

Shooting game - dont know mgs5?
Strategy - any chess game
Fighting game - Galaxy Fight

Enemy: MGSV
Ally: Uncharted 4
NPC: GTAV?