I would say without any doubt, halo 3.
Halo reach and halo 4 were great sure, bu they lost a lot of their identity with all the classes and the sprinting and such.

Halo 3 on the other hand, was just you, your gun and your own skills. No other bullshit advantages like jet packs or custom classes.

Including multiplats? Halo 3 and Red Dead, hopefully GTA V will be in my lifelong collection too.

Halo 3. Probably one of the top 20 shooters of this generation.

Shadow Complex was awesome! Hoping another one soon.

I'd ave to say Halo 4. Still the best Halo FPS.

My fav 360 game is still Halo Wars though. Had many, many a fun evening playing that on-line

Assassin's creed 2 , they kep saying you get to keep it forever

Lost Odyssey

The Gears Of War series.

I can't believe The Orange Box hasn't been mentioned yet.

well my collection right now;

All the halo games
Dead Rising 1
Lost Planet 1
Deux EX
Red Dead
Dead Space 1
Orange Box
Batman AC
Bioshock 1
PGR 3
either forza 3 or 4. take your pick.
AC2
Skyrim
Gears of war 1

and what I would say is the final bit of the puzzle and the crowning achievement for this gen; GTA5

Halo 3 was very well done and Test Drive Unlimited are still my favorite.

Borderlands 1 was great standout too.