PC is definitely #1. I'm not a WASD fan myself, so I use controllers for most games (except Portal 2), but I find computer gaming to be more convenient. I can go from playing Borderlands, to Portal 2, to Terraria, to Minecraft without having to do more than click in a few spots. If I wanted to do that on a 360 or PS3 (neither of which have Terraria or Minecraft, but that aside) I would actually have to get off my lazy ass at least 3 times.
Games can be bought on Steam and downloaded immediately at any time - from the convenience of your own home. Also, it's easier to pirate games on PC, if you so choose.

With Steam, your games are with you at any computer with internet access. With the PS3 and 360 you need to carry disks with you.
Next, I'd have to say the PS3. I like the controller better. Also, online is free.
360 comes in last, although, I don't hate it, it's just the lowest out of the 3 current best in my opinion - one of them has to be it.