Revan was more powerful than most, as per Kreia's comments, but that - of course - spoke only to his potential. Revan is much like Anakin in that regard, because it's unclear whether they ever reached their potential.
I don't see Revan as most powerful jedi ever, though he was probably one of the most gifted in his time, and maybe the one with the most potential. Revan was powerful, sure, but you can say the same about lots of jedi. The major difference between Revan and the others, however, is that Revan is YOUR character... And when people talk about how he (or the exile for that matter) are the greatest ever, I'm always uncertain about whether they are talking about the characters as part of the universe, or about them as extension of their own ill-hidden narcissistic fantasies. Not because I see people as particularly self-absorbed, but Revan does have the ability to pull that out more than most characters because he did have significant powers, but also due to his background. Ask yourself this - did you say to yourself "Wow, my character was the Dark Lord of the Sith" or did you say "Wow, I was the Dark Lord of the Sith"?