I have both the PS3 and the Xbox 360.
I honestly don't play the 360 that much, and the PS3 I purchased entirely for its Blu-Ray capability (well, that and the faint hope that worthwhile games might come out for it someday).
For the 360 I have:
Import Tuner Challenge. The reason that I bought the 360 (seriously). I love this game, but I realize that most people think it's crap.
Rainbow Six Vegas. Also available for the PS3. I don't think it's as good as Rainbow Six Three or R63: Black Arrow for the original Xbox, at least as far as the single player experience goes. It was still pretty fun to play, though (I like all of the added weapon customization options, as well as the ability to finally pick up weapons off of dead enemies), so I don't regret purchasing it.
Tomb Raider:Legend. Xbox exclusive (well, at least it's not available on the PS3, I think that it's available for, like, every other system in the world, though). I bought this one in a fit of nostalgia (I loved the first two Tomb Raider games). Sadly, it turns out that I no longer enjoy frustrating platformers, so this one sits on the shelf, mostly unplayed.
Marvel Ultimate Alliance. Also available for the PS3 (but I think there are more characters available for the 360 via downloads). I was on a huge comic book reading binge when I bought this, and I have to say that I really enjoyed it (I played through it twice). Might not be nearly as enjoyable to those who don't find it inherently cool to play as Moon Knight and kick the crap out of people with your nunchucks, though.
Mass Effect. Xbox exclusisve. Since this was touted as a tactial shooter meets an RPG, and I like both of those genres, this seemed worth a shot. Sadly, it disappointed in both its tactical combat and in its RPG qualities. I did manage to make it through the entire game, though, and mostly enjoyed it. The only thing I regret is paying full price for it.
Eternal Sonata. Xbox exclusive. A JRPG where you get to play as Frederic Chopin. This is one of those marvelously bizarre concepts that you can only find in a Japanese game. I purchased it recently because I was in the mood for a JRPG and this one certainly seemed to put the "J" in "JRPG". I'm still playing through it. It looks beautiful (its the closest I've seen to "playing an anime"), but I find I like it better in small doses due to the combat being kind of repetitive and boring.
Forza Motorsport 2. Xbox exclusive. I picked this one up because I was in the mood for a new racer after playing Import Tuner Challenge into the ground. Yeah, turns out that hyperrealistic racing games are not my thing. I played this for, like, an hour before throwing down my controller in frustration and I haven't tried it again.
