All of the above?
"Above all others"? Oh, well then.
PC, followed by Sony, then Nintendo, with Microsoft bringing up the rear.
As for why, well, I like the slightly less limited "freedom" of the PC. If I want to play my games with a controller instead of keyboard&mouse, I can. Hell, with a bit of work the PC supports almost any kind of controller; I use my PS3 Dualshock pad.
Also because I can easily go back and play older games like Sonic Adventure DX or Star Wars: Knights of the Old Republic or X-Men Legends II, Warcraft II or Starcraft, Star Trek: Birth of the Federation, Betrayal at Krondor, Fable, and (aside from availability for purchase if the situation might call for it) the only limitation on my ability to play them is my ability to tweak and replace files that might not be compatible with new operating systems. Sure, there's an amount of backwards compatibility on the consoles, and it does cover many of my most favorite games which are the ones I'd actually feel like ever playing again, but it's still not the same breadth.
Lower pricing and regular sales of games is a plus as well, of course. I like having physical media, but there's no way I would've ever purchased even half of my 140+ game Steam collection at $30+ per game.