I have to admit I've not purchased an XBLA title since Marathon Durandal, and tend to play most of mine before release though Partnernet. But I do regularly revisit the original classics on my standard 360.
Bejewelled 2 stilll keeps me quiet for hours on end, and I'm a massive Geometry Wars addict - though after cracking the million point mark, for some reason it won't be kind to me anymore...