Not sure there is a 'smart' fish. I think that people think some are smarter than others because: 1) some are more responsive to us than others, begging for food, etc., 2) some appear to be doing 'complicated' things like building burrows, rearranging things to their liking, etc.; and 3) some seem to do things which appear 'logical' to us and we attribute this to intelligence rather than instinct. Having said that, fish with 'character' like blennies, clowns, jawfish, make a tank more interesting to me. Not sure they're smarter but, they're more interesting to watch their behavior.
Matt