I use ctrl in almost every game. The only game so far I don't have my crouch bound to ctrl is Crysis 2, because it is really difficult to sprint and press ctrl at the same time to perform a slide.
Ctrl is the classic. That's like "What mouse button to you bind to shoot?" If it's anything other than M1, you are a freak of nature. Same with crouching.
I picked Ctrl because I used to use it. I have a Logitech G13 Gampad now, so I leave all the game defaults the way they are now and just program those keys into my gamepad.