Super Mario 64 DS (NDS)
P.N.03 :lol (GCN)
Capcom vs. SNK 2 EO (GCN)
Dynasty Warriors 2 (PS2)
I'd say any PSP game that uses the analog "nub" as it's primary means of control. That nub is like the worst thing that could happen to handheld gaming if Sony doesn't plan on moving it on their next handheld, much worse then the touch-screen. Though Virtua Tennis seems okay, but I haven't been able to get my friend's PSP long enough to really decide that yet. There are also many DS games that feel quite uncomfortable, especially SM64DS, but there are also games that fit it peferctly, I have yet to encounter this with a game on the PSP (VT might be the first ime).