Gears of War (franchise) - Playing through on Insane with a buddy. I've played through both games several times, and I'll probably do the same with part 3. I've played a ton of multiplayer (Annex and Warzone) and Horde too, but the co-op is still the best part of this game.
Virtua Fighter 5 Online - It's the best fighting game ever made. Online. Easy.
Super Street Fighter IV - It's the second best fighting game currently available. Online. Easy.
Team Fortress 2 - Simply put, the single most FUN shooter I've ever played.
Halo: Reach - The shooter I take most 'seriously', if that makes sense. I explore every nuance of the combat, the armor abilities, the maps. It just feels so deep. I feel like a learn something new and experience something different every time I play it.
Burnout Paradise - Just so much to do, and so much fun doing all of it. The racing is fast and furious, the group challenges are cooperative insanity, and Criterion just kept adding more modes. Marked Man is amazing.
Need for Speed: Hot Pursuit - A bit more focused than Burnout Paradise, but still just as fun. Races are all about knowing the track, and nailing the competition, no need to worry about taking different routes. Beautiful, fast, awesome. Made the NFS franchise relevant again to me.
Modern Warfare (franchise) - For 6 months or so, COD4 was the ONLY game I played online. I was fully addicted. Now I find I'm pretty tired of the formula, but I can't ignore just how much time and energy I sunk into that game, and how much fun I had. Spec Ops ensures that Modern Warfare 2 stays in my 'recently played' pile forever.
Assassin's Creed Brotherhood - Another case of a game owning my life for a relatively short time. My addiction to the game has faded since I hit level 50...which I guess (sadly) validates the whole prestige system in CoD, but I still throw the game in for some matches of Manhunt or Alliance with some buddies. Such a cool, unique, satisfying and laugh-out-loud fun game.