Immediatly coming to mind are Pacquiao, Marquez, and Gamboa when he decides to turn on the cyclone is pretty damn good too.
Also surprised nobody has said Sugar Ray Robinson.
JMM might be the best today. That Katsidis fight was a brilliant display. It's hard for me to make a list of the best I've seen. I'd have to sit down and think about it. I'm a bit too tired, haha. Good thread though. I love good combination punching.