Until yesterday, my 2 favorite Achievements were getting all 500 Agility Orbs in Crackdown and getting the Wax Off Achievement in Geometry Wars 2. But it will be difficult for anything to top my first Achievement for 2009; I have never exerted so much mental and physical energy to get 20 Points as when my wife and I played Rock Band 2 for literally 10 hours in a row yesterday, doing The Endless Setlist. I was playing drums on Hard, but my wife was playing guitar on Medium and so that's all we got credit for; that said, she saved my ass repeatedly during the last 3 or 4 songs and so I wouldn't have been able to finish it without her.
(We got five stars on pretty much everything right up until those last 4 songs, and then we just got killed left and right. I think it kinda sucks that you spend so much time playing all sorts of cool songs only to finish with a bunch of speed-metal sludge; I understand why it's there but I'd much rather finish with something a little more musical.)
My arms and legs are totally sore now, and I think I'm going to take an extended break from playing drums. But it was totally worth it; my wife and I climbed a mountain together, and that's a pretty awesome way to start a new year.