I'm a dedicated Virtua Fighter player. My blog Goh Notes can prove the dimension of this dedication pretty well. Currently, AM2's newest offspring Virtua Fighter 5 is what I usually dive into for the pure pleasure of what you get out of it.
Of course, for a good multiplayer shooter I always load up Valve's Left 4 Dead. Also, I took the time to write a comprehensive SuperGuide for this cooperative zombie masterpiece on GiantBomb.
My newest discovery is Rare's Banjo-Kazooie: Nuts & Bolts. I never heard of Banjo before, but having played this new spin of a platformer makes me want to long for more.