Half Life on a keyboard?! That's fucking crazy. I still think keyboard and mouse are ideal for any FPS. Other than that, a gamepad is probably ideal for everything else. It's not really an issue now because the 360 controller is natively supported by most modern PC games, and it's easy to use it with older games as well.
Call me insane, but I found SMB easier and more comfortable on a keyboard, and I finished the game with plenty of A+'s
I always thought turn based tactics games (Front Mission, Final Fantasy Tactics) would be fucking fantastic on a touch screen... they're okayish, I guess. Not so bad, but not heavenly. Is the Vita multitouch? If so, this might be awesome for that type of RPG, and finally consoles will have something superior to Mouse+Keyboard on some strategy and RPG and Tactics-type games.
If it's a retro stuff, it could be better with controller. Zero Tolerance (FPS), RPG (any JRPG of 90s), Strategy (Dune II) - they're all fine without mouse and keyboard. Well, it's pretty obvious what 2D/3D FPS with aim are best with PC controls. It's fo fuckin' hard to play Duke Nukem 3D on Saturn without analogue pad. Not familiar with modern console shooters, but played Resistance Retribution on PSP and deleted it after some minutes of playing - because if its controls.