Borederlands
Closely followed by L4d2, Batman AA and Red faction. I've not got round to playing COD:MW2 yet, though I'll borrow it from my brother for PS3 when I see him next - though perhaps that statement says more about the game than playing it ever could...
To all those whining about L4d2 and COD:MW2 being rehashes of the previous version with a few new additions, isn't that what a sequel is? Excluding sequels which actually came out more than a few years after the originals of course.