I try to leave him out of most of my mentions to avoid being looked at as a nut-hugger, but Whitaker was definitely one of the best at this. His jab in particular, which is why I rate it so highly. The way he was able to sneak his jab through an opponent's guard while simultaneously slipping out of harm's way was brilliant.
Qawi deserves a shout here. Look at what he did to Rossman and Saad Muhammad, even L. Spinks. Heck, Holyfield's guard was busted in those middle rounds in their first fight.
A lot of good mentions in this thread. While I don't feel he's the best at this, I think Chavez deserves an honorable mention. While Chavez preferred creating openings in an opponent's defense by using blows in combination, his punching accuracy meant that sometimes he'd just send one or a series of blows right through someone's guard. My personal favorite example is when he mauled Haugen in the last round of their fight. Check it out starting at about 1:02, when Chavez wobbles Haugen with an overhand right then pounces on him. http://www.youtube.com/watch?v=Kxu8frKEB8E
Max Schmeling deserves a mention. If you didnt drop the left he threaded his right hand past it or flicked it aside. Nobody seemed to be able to stop that lead right hand ariving.