Pretty much anyone who complains about Halo 3 being "just OK" is dealing with sour grapes for not being as good at it as they are at one of the previous two iterations. (If that is the case, they patched the melee system to be more fair a month or two ago, if that was your issue.)
I'm no ace at any of them, but I'm decent, and I can say it's the only one that really gripped me. The online skill ranking system, crazy stats on bungie.net, different armors, etc., provide good motivation to play through the game, unlock things, and try to get better so you can play against better competition online. The plot is a tad wonky, but the story mode on the whole is awesome to play through on legendary.
That said, I'll go with your top-four and ignore the ones you say you have on PS3 or aren't interested in, but after that I'll only list games I can personally vouch for:
1. Halo 3
2. Gears of War
3. Mass Effect
4. Bioshock
5. Oblivion
6. GTA IV
7. Marvel Ultimate Alliance
8. NBA 2K8
9. Prey
10. Phantasy Star Universe
11. Capcom vs. SNK 2*
12. Sneak King**
* Regular Xbox, but still awesome online through 360. DOA4 and VF5 aren't bad, either, but 2D fighters are still the best.
** Sneak around and surprise people with Whoppers as the Burger King.
Xbox Live has a ton of good downloadable games like Ikaruga, Castlevania, Undertow, Aegis Wing, Contra, N+ (a ninja game), Street Fighter II, Mortal Kombat, and basic stuff like Poker, Chess, Hearts, etc., that are also worth looking into.
I recommend going to
http://xbox360.ign.com/index/reviews.html and then putting "9" and/or "8" into the drop-down window for "Show Score." That should cover all the good'uns. (GTA IV got a 10 despite its minor flaws.)
You're definitely going to have to let us know how you're going to get 12 games for $180, though.
Also, please keep your gamertag "Roy Hobbs" ... I don't know how you could go any other way.