PC, by far. It's the most powerful with the right components, you can do a hell of a lot more than just play games, the mouse and keyboard is the ideal control interface for strategy and shooting games, you can play almost every game ever made using emulators, and the internet "service" is unrivalled (we take for granted what Xbox Live users pay for, it's been around for us since before the 90s). With the right skills mod teams can enhance and completely alter games, creating free content. And since they are not bound by the fear of losing their investment, they are free to experiment with all types of weird and innovative gameplay ideas that publishers just aren't willing to risk. That's a great thing considering how much it costs to make a modern game, which can have hundreds of people working on them at a time.
The downsides to the PC are cost and the fact that you can't really play split-screen with one unless you have it hooked up to a large screen and you're playing a game that uses controllers rather than the mouse and keyboard. I play old school arcade and console games with my friends all the time.
