DLH's Left Hook or that bolo punch Ibeabuchi took Byrd out with. I love the Overhand right as thrown by Mosley and Lennox Lewis too. Hagler's Right Hook too.
A left uppercut thrown by a orthodox fighter or vice versa. I think people that manage to hit that punch with any kind of power shows some true skill. The way Marquez can throw it of a jab in a fluent combination from jab, left uppercut, right hand, left hook never ceases to amaze me.