Clearly Mohammed Ali's the biggest -- he's one of the biggest names ever. Who's the second biggest name at the moment? The name of the person, dead or alive, that the largest number of people on the planet would recognize, apart form Ali?
I'm split between Joe Frazier, Mike Tyson, Floyd Mayweather, and George Foreman -- though Foreman's really known more as the guy with the grill who also boxed.
If you mean right now, it's Mike Tyson and it's not close. If you mean all time relative to era, Jack Dempsey is second and it's not close.
Tyson Joe Louis is one of the most popular ever and even to this day many have heard him. During his time he was as big as Ali was during Ali's time.
In Britain, no one knows Oscar, but everyone knows Mayweather cause he beat Hatton. I guess Oscar's much bigger over in the west.
Pacquiao's less well known. There's a cultural issue. Black guys know Mayweather -- he's was symbol of pride and an example for them. All but a very few black guys I know know of Mayweather. Pinoys are a total minority in the UK by comparison. A corollary to this is that people are simply more interested in a black guy with volumes of chat than a Pinoy who doesn't speak good English. Another cultural issue is that Pacquiao's not as catchy a name. Simple, but it makes a huge difference. British news only really reports boxing where it concerns British boxers, and in that regard Mayweather wins again, because his fight with Hatton was better known and more widely publicized. This is the same reason Chisora is probably bigger than Pacquiao at the moment for his brawl. I honestly couldn't say how well known Pacquiao is. No one except the very occasional person at my university seems to know of him, but my university is virtually totally white middle-class, with a strong upper-middle contingent ... people know more jockeys than they know boxers.