Who's the greatest of all time?

Boxing has had fewer better days of late. With his pound-for-pound skill and compelling backstory -- so versatile is Pacquiao that, after stopping Cotto in the 12th round, he took the stage at Mandalay Bay to perform with his band, Calsada -- Pacquiao appeals both to a hip-hop generation and old-school fight fans.

Is boxing back? The New York Times declared in a headline last week "With Pacquiao-Cotto, Boxing Is Ready for a Rebirth, Again," but only a fool would suggest that the sport will ever again reach the level of interest and relevance it enjoyed in its glorious heyday. Still, the sport produced two fights in 2009 that generated more than one million pay-per-view buys, assuming the Pacquiao-Cotto numbers come in later this week as expected. (All anecdotal evidence suggests the fight will exceed the million mark.) Earlier this year, FloydMayweather's comeback win over Juan Manuel Marquez drew more than one million buys, as did Pacquiao's win over Oscar De La Hoya in December 2008. The last time boxing had two fights that drew more than one million pay-per-view buys in a single year was 10 years ago when De La Hoya-Felix Trinidad and Lennox Lewis-Evander Holyfield both topped the mark.

Pacquiao's appeal bodes well for an upcoming megafight against Floyd Mayweather Jr. -- a bout that fans have been pining for ever since Mayweather announced his return to the ring. As Pacquiao's long-time trainer Freddie Roachrecently told SI.com, "It has to happen because boxing needs that fight. Negotiations are going to be tough of course, but I think it can be done at 50-50. The best need to fight the best."

With an eye toward the best of the best, SI.com asked senior editor RichardO'Brien, who has covered boxing for 20 years, to rank his all-time greatest fighters and fights. Below, you'll find links to galleries for our top fights and greatest pound-for-pound fighters ever. We've also ranked history's best heavyweights, middleweights and lightweights. You can weigh in with your thoughts here.

Our lists go back a ways and feature more than a few fighters from many years ago. After Saturday night's thrilling main event, there's reason to hope that some new contenders are yet to come.