There are literally dozens of knots that you could use while climbing but for obvious reasons there are certain ones that are not used. Would you use a granny knot to tie into your harness? I certainly hope not, because you’d probably die.
Even if you have all the gear and the knowledge to sport climb, if you don’t know how to attach yourself to the rope then what good is it? The general consensus among climbers is that the Figure Eight knot is the one to use. It’s very simple to tie in, and it’s about the strongest knot that you can use. When I say “strongest”, I’m referring to the force that the knot can handle before it breaks.