PS3 - Best Graphics, easy to use controller (the X360 controller is like holding a football), Playstation online access is FREE (Unlike live) and the blu-ray is a big bonus.
XBOX 360 - Has a bus load of games, has HALO (which isn't all that), GTA 4 episdoes exclusive (im getting that) and more people have it then PS3 (Might change after price cut, since PS3 slim sales have went up dramatically).
Wii - Good clean family fun, Mario (Best video game personality ever especially when Yoshi is involved

), the controllers are the life line for this system and Zelda is a Beast.
Thankfully I have all three!
I would have to say I use the PS3 the most because of the graphics, blu-ray player and the controlls are simple to use. One of my favorite this is that you can still play PS2 games on it as well (if you have one of the older models as PS3 Slim does not have backwards capability).
I would say I rarely use my XBOX 360. It does have great graphics and is generally fun to play, but I pretty much use it as a back up or if an XBOX 360 exclusive comes out.
The Wii is mind blowing fun and I have always loved Nintendo games. It almost takes you back to your childhood. I rarely use it since I play games like Mortal Kombat, GTA and more hardcore games that Nintendo doesn't offer.
To be honest, you really can't go wrong with any of these systems.
PS3 - Best Graphics/ Blu-Ray
XBOX - Wider selection of games
Wii - Family Fun/ Innovative controllers
It all depends on what your looking for!