For me it would be Tempest 3000 for the Nuon, or T2K for the Jag if I feel like slumming. They have that zen shmup quality that lets you turn your brain off and get in the zone.

Lately when I get that angsty "I have all these games and I don't want to play any of them" feeling I play Robotron 2084 on XBLA. It shares some of the same positive qualities as Tempest, but I don't have to get up from my computer chair to play it because I have the 360 hooked up to my monitor.