I would have to go for a Bomberman title. Not sure which one, and there are some I haven't played, but to me, Bomberman is the pinnacle of multiplayer gaming. I usually carry around 4 GBAs and Bomberman tournament, which is perhaps the ultimate.
Everyone has their own screen, multiple maps, full colour, and you can take it and play it anywhere. When I play 2 player with the CPU filling in the last 2 spots that's fun, but when I get 3-4 people, I think it's some of the most fun I've had playing games.