You should have separated a distinction between online and local, and if co-op also counts, but oh well.
For local, Halo 2 and the first game is my favorite of all. I passed many hours with friends over that mode. I also love the multiplayer of any Mario Kart (with emphasis on 64 and Wii), the crazy Co-op of New Super Mario Bros Wii and the simple fun of Wii Sports and its Resort sequel. I actually remember doing a favorite local multiplayers over 1up.
For online, so far it's only Team Fortress 2, but by god is that game lots of fun and so well done.