Definitely the new Geometry Wars! I must have played at least one million games of the first one and even bought a DS so I could get another fix. The only thing I am missing is a Wii and that version. With so many good new games coming out for the 360, I don't think a Wii will be in my budget anytime soon!
In the mean time, I am still having a blast with Geo2. I still have a few achievements to go, but I play everyday to try to improve my scores and to catch my friends. They are much better than I am, but that doesn't stop me from playing hours every day! The 360 rocks!