OK, wot I am thoroughly enjoying unconditionally on Xbox 360:
Fable II
Forza Motorsport III
SAW
NIER
NFS Undercover
NFS Shift
Fallout 3
Overlord II
I like loads but currently stuck on:
Bayonetta
Final Fantasy XIII
Eternal Sonata
Death Spank (shame, cos this, imo, is a great game)
Sonic & Sega All Stars Racing with Banjo Kazooie
In Sonic & Sega All Stars racing I'm stuck on a level where you have to race and avoid hitting obstacles on the track, I just can't do it. But there's still a lot more to the game to make it worthwhile keeping.
Entertaining but not in top league:
Sega Superstars Tennis
Super Street Fighter IV
Dragon Ace: Origins
KAMEO: Elements Of Power
Morrowind Elder Scrolls III (Xbox)
James Bond 007 Nightfire (Xbox)
Max Payne II (Xbox)
Doom II
Haven't tried or played enough yet to form an opinion:
Naughty Bear
Fable The Lost Chapters (Fable 1) (Xbox)
Assassin's Creed 2
Condemned 2
Tomb Raider: Underworld
Great games but I'm waiting til I get the converter thingie to play with mouse & keyboard:
Call Of Duty 3
Halo 3
Timeshift
Gears Of war 2
Absolute pants:
Onechanbara Samurai Bikini Squad (possibly the worst game in the world)
GRID
Blue Dragon
FUEL
Flat Out: Ultimate Carnage (actually, this one wasn't too bad but I sold it anyway)
Thrillville OTR
Oblivion: Elder Scrolls IV (After playing this through on the PC it just didn't seem to work on the Xbox 360)
Fairytale Fights (wouldn't run)
And a couple of fps's I sold but in hindsight would probably work well with the mouse/keyboard converter thingie, they both seemed like very good games but were just too difficult to play with a gamepad:
Gears Of War One
The Darkness
All of the above is purely my opinion, others may disagree. And also others will be better gamers than me and not get stuck so easily
I'm really quite disappointed with Bayonetta, I got stuck on third level, because up until then it was very enjoyable.
I'm also stuck on a few PC games, like Batman Arkham Asylum and Metro 2033, maybe I getting too old for this stuff
And finally, for PC, I'm going to buy Borderlands with Expansion pack in GAME store - two for £25.00 offer and also fps Arma II: Operation Arrowhead. And when Mafia II and Starcraft II come down in price I'll probably have some of that as well.
Phew