QUOTE(wakingdream @ Feb 9 2008, 03:40 PM)

Not necessarily the best, but two of the the most famous are definitely Copacabana and Ipanema. That being said, there are tons of other fantastic beaches to choose from. Costa Verde area aka Green Coast, between Sao Paulo and Rio is very beautiful but can be packed to the brim with package tourists. In Ceara State Jericoacoara Beach is really nice and not nearly as developed as some others offers respite from crowds. Bahia State has a huge number of beaches as well. Plus you'll get a chance to explore more of brazil's colonial past. It's a very nice area to visit.
As Martin asked, which area(s) are you planing to visit?
Buzios has 23 amazing beaches , I think some of this are the best of Brazil ( geriba , Azeda, João Fernandes and Ferradurinha)