The NBA was formed in 1927, and it became the WBA in 1962, so therefore is is the oldest world boxing organisation by far. The WBC was formed the next year. It's hard to say who's the most credible these days, I'm not super up on the boxing potitics, prefering to just follow the fights that interest me, but The WBA does superchamps now don't they?meaning they have multiple champs in one division. Add to that the IBF being monitored by the government for being corrupt, and The WBC nearly going bankrupt for illegally stripping a guy of a belt he won, and you could say the WBO is the most credible, as they just do what they're supposed to, put on fights. They're not, but you could see it that way.

And no, I've never heard of someone having more than 6 belts at once.