PS3 beats out 360 by a narrow margin. It seems the games sometimes look/play/run a little better on that system (with some exceptions, such as Bayonetta), plus it has blu-ray, and I think the trophy system is more fun than the 360's achievement system. XBL is better than PSN (due to their Indie Games/Community Games/XNA support, and the support for XBL parties), but I don't play enough multiplayer to really notice. Also my 360 was an early model so it only has a 20 GB harddrive, which is another reason I usually go with PS3 over 360 games. It (the 360) does have backwards compatibility for the same reason though, which both systems discontinued in later production models. This opinion may change though once they release Move and Kinect... it could go either way, but right now I'm betting Kinect is gonna be more fun than Move (although there seem to be better Move games coming out in the first set of releases, in particular RE5, Killzone 3 and Heavy Rain).
Wii was my favorite system for about a week after I bought it, and I've barely played it since (part of why I'm skeptical of Move). Computer sounds great on paper but I never really got used to the keyboard and mouse set up.