My professor is a fish neurobiologist. He always says that the cichlids are very smart...for non-reef fishes. He's thinking of working with them because they're so easy to collect/breed. After performing a lot of fish brain dissections myself, it's true. Triggerfishes have really impressive brains....while the brain of a frogfish (angler) is really, really tiny.
Bichirs are dumb too. Without knowing much about its brain though, I would say my TT eel is pretty smart. However, oscars may take the cake in my book.
Size isn't all of it though, it depends what it's for. Some mormyrids (elephant-nose fishes) have a brain-to-body weight ratio larger than humans! However, it's mostly to operate their electrical organs. Usually intelligence is measured more by the telencephalon, which is like our cerebral hemispheres (the things you see when you look at a human brain).