One of the best (if not THE best) is Porcão Rio's on the seaside of Aterro do Flamengo. Gorgeous place and wonderful views! Perfect, delicious meat and salad/sushi buffet! But it is on the expensive side... I usually save this one for my birthday... ;))
I also like very much Barra Brasa at Barra da Tijuca, but it is also expensive and far from Zona Sul and the city center... You would need a car.
There are cheaper ones but the meat is not perfection... You can try Carretão in General Osório square (praça) in Ipanema. It is decent (but some friends of mine just refuse to go there because they think it is not "proper churrasco").
Churrasco here is like a science! It's not only the matter of the best meat but also the right way of salt, roast and cut it!
Anyway, churrascarias are all-you-can-eat places so even with the great, expensive ones you can starve before and give then a run for their money! Hehehehe! almost 6 years ago