Xbox is my favorite for emulation I think, PSP is awsome for portable emulation obviously though. A mate has a GP2X which does have superior emulators for some systems but the screen, dpad etc just dont make the grade.
No GTA either
Xbox has perfect SNES, Genesis etc emulation on the big screen. Pretty good N64 emulation also.
Xbox Media Center is amazing too, everyone should have a modded xbox.