I've used all three consoles, while I own a 360 (and an SNES! ). IMO, The PS3 may have better graphics, but Resistance and Motor Storm are the only games exclusive to that console worth getting. The Wii has the "exercise" and compatibility with previous consoles plus and some fun games, but the novelty wears off after a while. The 360 IMO is the best, for sheer volume of games. As well, the "Platinum Hits" series games work on both Xbox and 360. Now that the kinks have been worked out, we see a decent console with excellent graphics at an affordable price.
My choice? The 360. It is, without a doubt, one of the best purchases I've made. That and the SNES.