Well I haven't owned a 360 but I've played most of it's great games. I haven't had a chance to play Mass Effect yet so I can't comment on that, but IMO I would say Viva Pinata or Dead Rising.

Bioshock is fun, but has little replay value and you are already getting a shooter.
Gears is fun, but again you are already getting a shooter.

So yeah, Viva Pinata is awesome and it's one of my goals in life to get more people to buy that game (recently bought it myself on PC). Dead Rising is awesome as well for many many reasons. Anyway, that's just my two cents.

Um... most likely Mass effect i don't mind saints row either but for exclusives, but if we are talking shared here... guitar hero 3 and bioshock

Oblivion, although Mass Effect is running a very close 2nd at the moment.

1.Mass Effect 2.Blue Dragon

Dead Rising! While I don't play it as much anymore, the wow factor of having hundreds of zombies on the screen at once never got old. It pulled off the zombie feeling perfectly. I didn't buy Halo 3 :P

Also Forza 2. The customizable option for cars alone make it a gear head's paradise. Like any racing game, there's enough depth to make it interesting for months and years if you want to dig that deep.

Now that the superior PC version of Gears is out I sold my 360 version, or it would have been #2 on my list, and since it's rumored Mass Effect will be coming out for PC, I'll wait till then and get the better version. I can wait a year, I hope. It looks amazing from what I've seen so far.

tie between mass effect and bioshock.

Oblivion is my second fav game on the 360 even though it does not have a native resolution in HD.

Dead Or Alive 4 just awsome.

i liked gears

Bioshock or Mass Effect or Orange Box.

Halo 3 is not even in my top 6 360 games.