There are a lot of games where the AI isn't really held back because of how the game plays. So in these games, the AI is actually pretty good.
Most fighters have good AI on harder difficulties because it's coded in to predict your moves. You ever play Street Fighter 2 on the hardest difficulty and the moment you do something the computer always happens to block or counter whatever you do? It's because that single frame that you press the button, the computer knows what you did and the AI is coded to do this in order to counter. Atleast that's what I think they do.
For shooters I think the Uncharted games have the best AI.