well i am lucky enough to have grown up in the middle east (in dubai) we used to go over to the east coast of the uae and go snokelling.. and the fish were amazing! you could never imagine so many different colours and types of fish, swimming with turtles and sharks and everything i love it!!
in thailand we snorkelled at phi phi island, and the fish actually ate banana out of your hand.. ! its just a mass of moving colours.. so many fish you couldnt' even pinpoint one that is more colourful than the others!
For me it would have to be something like my Peacock Bass or arowana but i'm sure there is probably a African Cichlid or two that would know spots off them, some of the rainbow fish have some pretty fantastic colours too.
oops didn't notice the "freshwater" bit.. doh!
had a look at those peacock bass, they are really beautiful!
i would agree that there are some stunning aftican cichlids around though. i would say that they are probably the most colourful freshwater.. although like xamdarb said discus are also beautiful.. especially the blue ones..
this is almost as hard as choosing a marine fish!!