Alternating between Easy and Medium on NFS: Shift has reminded me of just how atrocious the AI in some games are and has made me want to make a list of the worst game AI I've ever had to deal with. The sad part is that close to half the list would likely be NFS games. And knowing me (whenever I get the time), I likely will make that list. From the get-go I know that these are almost guaranteed inclusions:

NFS: Shift

NFS: Pro Street

Burnout 3

Midnight Club 3

DBZ Budokai 3

Come to think of it, I've only seldom played any games that have been legitimately challenging with frustrations being kept at a minimum. Halo, Halo: ODST (for what I've played), Uncharted, Forza 2, FEAR (speaking of which, I must be the only one who thought the sequel was just lousy-I didn't finish it but that's because it was so bland and unremarkable) and Call of Duty 2 are the only games I can remember playing that had AI that put up a legitimately tough but fair fight.

I guess that's a big reason I usually prefer goal-oriented games and modes like Tony Hawk's Pro Skater 3 and drifting in Underground 1 and (generally) Shift.