Oh god. My preferences are hugely fickle buuuut, I primarily split my time between the 360, PS3 and PC as it currently stands (with some PSP love on the side now that Tactics Ogre just came through my letterbox).
I've actually been equally dividing my time as of late between the PS3 and 360. I've been ploughing in a shitload of time on the God Of War Collection and Yakuza 3 in particular for the machine and I guess this outlines the reason why the PS3 is such a robust staple of my gaming diet; I love many of the exclusives for the console and find a great deal of compelling and enjoyable experiences in them for the most part. Games such as Uncharted 2, God Of War 3, the Yakuza games and Demon Souls really validate the ownership and repeated use of the machine in my eyes. The PS3 is also my de-facto choice for fighting games too - simply because A) The default pad IMHO is naturally better suited to the accurate sweeping motions demanded by so many contemporary fighting games and B) Because... well, i'm a tight-ass and can't be farked to buy a fightpad/stick.
As far as the 360 goes, it's this platform that gets pretty much all of my online love. I have a large number of friends who play a lot of multiplayer FPS's on the console and whilst the PC would *normally* be my first choice for the genre, with the 360 I find the Live service highly accessible and very robust for the most part.
Additionally, while I find the PS3 pad to be superior for fighting games, for me at least, I find that the 360 commands a similar advantage with it's pad for FPS titles. Specifically it's the positioning of the analogue sticks which just makes moving, strafing and shooting a lot easier than trying to pull it off on Sony's machine. The other thing which keeps the 360 high in the attention stakes for me is the hugely substantial Xbox Live Arcade offering - with titles like Limbo, Trials HD, Shadow Complex and many, many more being a definite highlight for me.
And finally my PC. Up until recently my PC was pretty much the go-to machine for World Of Warcraft and nothing else, but, with a fairly solid line-up of games coming in the Witcher 2, Battlefield 3 and Diablo 3 I can certainly see it becoming a mainstay in my gaming diet pretty quickly indeed. Another reason I like my PC as a gaming platform, is the wealth of gaming experiences available on Steam, not to mention the fact that in regards to multiplatform RPG's such as Dragon Age, Fallout and TES, the PC version almost always outperforms it's console counterparts and retails at a much more wallet friendly pricepoint.
I could ramble on for hours about this, but I guess at the end of the day you could say that depsite my appreciation of multiple platforms, I remain a devout fanboy. A rabid one. I am a fanboy of good games with the lumps of heated plastic which power them coming a distant second in my gaming preferences and as such I go wherever the good games are and this far, the 360, PS3 and PC have satiated my need in this regard quite suiccently indeed.
Roll on E3 and beyond!