If you're looking for simulation, then Pro Evolution Soccer is unmatched in my opinion. Recently, however, I've been leaning towards pcik-up-and-play arcade titles that favour fun over realism, and Mario Tennis for the N64 is absolutely perfect in that regard. Five-set doubles matches are responsible for some of the best multiplayer fun I've had in ages.

EDIT: Oh, and NBA Jam. Seriously folks, just play it.