1) Gears of War 3 - Versus MP. The combat mechanics can't be found in other games and is truly unique. The dancing, wall sliding/cancling. It's great. Kinda sad that execution didn't become the standard gametype. The campaign was disappointing though(Halo:Reach shits all over it).
2) Mass Effect 1 - almost didn't list this, because of what happened to the rest of the series. I didn't forget how amazed I was with this game. I finally got into the wrpg's because of this. Tbh I've played better wrpg's by now, but the initial impact is unrivaled.
3) Halo:Reach - 4p campaign on Legendary is simply blast. Pure and simple fun.
This kinda makes it seem like I only love 360 which is hardly case, but I tried to think of it in pure pleasure terms. Portal 2, Persona 4, DX:HR, BF:BC2, Tera Online(till you hit the level cap), Crysis, Lara Croft and the Gaurdian of Light, Bayonetta, Mass Effect 2, Xenoblade Cronicles, Street Fighter IV. I think these games are just as good. The Gears franchise still has the top spot though. I'm not hyped for Gears:Judgement though, franchise fatigue maybe. :/
Lack of SMG2 is explained by that I'm not into platformers. I did enjoy it though.