I've played a TON of great games in the past few years, but honestly... Left 4 Dead stands out for me. I put in so many hours on that game, and the multiplayer experiences for it are second to none. The original Gears of War stands out for me as well, because I dumped way too many hours into the multiplayer portion of that game as well. But overall, Left 4 Dead had some of the coolest moments & craziest encounters. What about you?
Holy shit, Team Fortress 2. That game made me laugh every time, either from the glitches we found, or the hilarity of the community.
I'd have to agree with WhoIsDo. Left 4 Dead was a game that I just spent countless hours on with my friends. So many memories of just messing around and having fun, as well as playing the game and trying to beat that game on expert (still can't do it). Wasn't a huge fan of L4D2, but maybe because the original was still fresh. Hopefully if/when the third one comes out, it'll have been a while for me to put many more hours into it!
Probably the Uncharted games. I can play them at any given moment, and still enjoy it. God Of War is also a franchise I never tire of.
Gears of War. That was my all time favorite franchise on the 360. Played all of them religiously, I was even a Reaper on Judgment.
Personally, I love the Sly Cooper series. One of my favorite series EVER. Played all of them (except the recent one, which I need to get).
Halo 2 on XBC(XboxConnect) Played Halo 2 there for over 2 years and I've never had so much fun and good memories while playing with people around the world.
Fallout 3. Months of time and lack of sleep. That game even changed my eating habits. Still have display cases dedicated to all the collectibles. Skyrim. Lost in the story and the freedom. Wonder how my new house is going...got to go check COD4 was my FPS addiction and I agree, the best FPS still. BO1 was close second.
Streetfighter 4 in any of its iterations, I absolutely love the complexity and competitiveness of it. It's a game I always have time for, and it's been a constant for me and my friends for years now. I still nip onto my 360 just to play it when I have the chance. Actually going to a national tournament for it at the end of this month which I'm looking forward too.
Most recent are Fire Emblem: Awakening & The Last of Us. (sorry, I'll be repeating quite a few things from other topics) War Thunder was fun too. PAYDAY 2 I would include, pre-patch - the most recent one just made things too hard. The year before last, some friends and I had a blast playing MP Ghost Recon: Future Soldier. Under-hyped and under-appreciated I think (but then I know some hardcore GR fans that say they butchered the series). Earlier in the previous generation, Halo 3 stands out for me, again because of the 4-player coop and having a dedicated (but casual) team in MP. Forza 2 and 3 round out the list with some pretty awesome times setting up our own championships.
1. Half Life 2 series 2. Mass Effect Series 3. Left 4 Dead 1 and 2. 4. Battlefield 2142 In that order!