Asterisks because I know Muhammad Ali is considered the greatest. But can't we have greatest in other sports? I'm going to make an official list of who all can be called the greatest:

Basketball - Michael Jordan

Baseball - Babe Ruth

Golf - Tiger Woods

Cycling - Lance Armstrong

Running - Haile Gebrselassie

Soccer - Pele

Tennis - Roger Federer

Volleyball - Karch Kiraly (I have no idea on this one, but the name "Karch" is pretty cool, isn't it)