*** Disclaimer: the following post is simply my opinion based on my experience using the different platforms. ***
Xbox is my favorite. It has everything I want in gaming, with a certain ease of use. IMO, it and LIVE have the best UI in the industry right now. The online infrastructure is better than their competition (by far), and they release games that you actually get to play (instead of watching a movie interrupted by QTE's).
PC comes in a close second for me. I have a great rig, but I only utilize my PC for a majority of my RPG's and MMO's. Though even my MMO gaming may move to the Xbox if talk for the future holds true.
Playstation has dick that interests me atm, we'll see what happens in the future though.