Holmes has to rank up there -- built his dream house and paid it off, so no mortgage and still lives there, built an office building and signed long-term lease with the federal government (post office rent checks don't bounce), etc., etc. Also, IIRC, he negotiated to own the rights to the video/film of his fight with Tyson. Not many guys think of that, and I believe he cut his own deal on that one. Bonecrusher Smith was the first heavyweight champ with a college degree, in business administration. He has established a charity that funds scholarships for disadvantaged youth and has also worked in college athletics administration. So he's no dummy.
For ring smarts and boxing related smarts you can't go past Sugar Ray Leonard. He just plain outsmarted some guys including Marvin Hagler.
Out of the ring? Sugar Ray Leonard, Juan Manuel Marquez, Sergio Martinez, Miguel Cotto, Marvin Hagler, the Klitschkos, all seem sharp and able to understand complex ideas and express themselves well. Tyson, recently, has shown more intelligence in his conversations now that he's mature and clear-headed.
sergio martinez, the klitschko brothers, and ray leonard, inside the ring pernell whitaker, floyd mayweather jr and willie pep
Czyz always struck me as a guy who THOUGHT he was the smartest guy in the room. I'd probably go with Archie Moore just off the top of my head.