As of right now I play my Nintendo DS the most. However I think that has to do with the fact I have an M3 for my Nintendo DS so having every single game at your fingertips helps you play it more.
However my XBOX 360 is the best for just sitting at home and gaming. Most of my friends have an XBOX 360 and I only know one person who has a Nintendo DS and they just recently got the Nintendo DS. So the XBOX 360 is a lot more while playing with friends online. Plus with the XBOX 360 version of Naruto just released (I have to wait until Sunday for BestBuy to get it in :(), I have a feeling I'll be playing my XBOX 360 a lot more especially since I'm on the last boss for Zelda: Phantom Hourglass.