Must play blockbuster titles:
The Orange Box
BioShock
BioShock 2
Lost Odyssey
Bayonetta
Brutal Legend
Splinter Cell: Conviction
Dragon Age: Origins
Dragon Age: Origins - Awakening
Batman: Arkham Asylum
Forza Motorsport 2
Forza Motorsport 3
Blazblue: Calamity Trigger
Red Faction: Guerrilla
Dead Space
The Chronicles of Riddick: Assault on Dark Athena/Escape From Butcher Bay
Fable II
Halo 3
Halo 3: ODST
The Darkness
Crackdown
Grand Theft Auto IV
Grand Theft Auto IV: Episodes From Liberty CIty
Saints Row
Saints Row 2
Bully: Scholarship Edition (If you didn't play it on PS2.)
Ninja Gaiden II
Dead Rising
Niche titles I enjoyed:
Raiden Fighters Aces (Classic shmup gameplay!)
Raiden IV (See above description.)
Deadly Premonition (A hilarious horror game with VERY dated gameplay, but a very funny and interesting story and cast of characters. Only $20! This is a love it or hate it game, though.)
Earth Defense Force 2017 (Hilarious and quirky Japanese budget title.)
Silent Hill: Homecoming (I enjoyed this one, but it's very hit or miss, thus I put it in the 'niche' category.)
Ninja Blade (A good hack n' slash, but it has lots of QTE's, so this is also hit or miss. I personally think the QTE's are well done.)
Blue Dragon (Not quite as good as Mistwalker's other title, which is Lost Odyssey, but it's an enjoyable old school romp from JRPG legends. And it's super cheap these days!)
Wolfenstein (A very solid and somewhat old school FPS. The single player is fun, but the multiplayer sucks, so don't expect much out of that portion.)
Stoked: Big Air Edition (The best snowboarding experience you can have on any gaming system this generation!)
Divinity II: Ego Draconis (This one isn't for everyone and has some tech issues such as framerate dips, but its enjoyable if you can look past its faults.)
Risen (See Divinity II description.)
Sacred 2: Fallen Angel (See Divinity II description.)
The Saboteur (A beautiful and flawed open world gem. This is much better than Pandemic's uber disappointing Mercenaries 2.)
Wet (A stupid and corny Grindhouse style game with a badass protagonist. Animations and graphics are dated, but it's a fun blast at a budget price.)
Halo Wars (Not for everyone. It's a beginner RTS game, but it has simplistic, easy to use controls and gameplay. It also has some amazing cutscenes.)
Dark Sector (This is a B game through and through, like many of the games in my niche category, but it's a decent way to spend a boring 8 hours. And it's a few bucks these days.)
Rise of the Argonauts (I found this enjoyable. It's like a buggy and unpolished Mass Effect in a mythological greek setting. This is also a love it or hate it. There are many flaws to look past, but the story and dialogue trees are great here.)
Banjo-Kazooie: Nuts & Bolts (A pretty huge departure for the franchise, but it's very enjoyable nonetheless.)
Kameo: Elements of Power (One of the few true "adventure" games on the Xbox 360. I really enjoyed this one.)
Mirror's Edge (A first-person parkour/platformer set in a beautiful and rich setting. Some gameplay quirks can frustrate, but this one is quite unique and worth trying out at the very least.)
Prey (An old school sci-fi shooter in the vein of classic id titles. It's linear and dated, but it's fun and cheap.)