I always thought that De La Hoya could throw very good combinations. Trinidad was lethal when he let his hands go, as was Barerra. Direll (as much as I dislike him) can throw some flashy punches as can Khan. My vote would be Roy Jones.
Today? Pretty Boy Floyd... no competition. All time? Harry Greb is said to have had the best combos in the history of the sport. If you read the newspaper accounts of his fights, that's what transpires.
For right now, JMM and Pacquiao, whose combos are underrated because they often look more flurrying than they really are. Hasegawa and Soto can both put 'em together too. All-time, damn, SNV, SRR, SRL, Barrera, Orlando Canizales, Floyd Patterson, Mike Tyson, there are so many badasses.
Right now, I go for Pac. His combinations were on full display against Marg -- offensive flurries around the guard, straight through the guard, and mixing it up from body to head, all in one combo. His hands are super quick, but more importantly, his combinations are just very fluid and seem so natural. Other fighters have a tendency to look a little stiff or robotic, even if they're really fast (Khan, for example). Pac manages to use combinations which combine speed with fluidity. They're much sneakier than he's given credit for.
Pac is the best I've ever seen put together punches. All with power, speed, explosiveness and heck, he throws 10 punch combinations...