I would say that many fish are far more intelligent than oscars.
CFC used to have archerfish which could "shoot" him with water through a tiny gap at the top of the tank when they were hungry, knowing he would go and get the food.
I have had a few marine fish recognise that I feed them and so after an initial display at a distance ignore the wife once she is nearby.
Then one can move onto the predatory tactics of some sharks (such as the Great White) which periodically peek across the sea from the surface to check what is going on before submerging again to launch an attack.
If one moved into inverts then octopus rank fairly highly.
Of course, in reality, one must first define intelligence before deciding which is the most intelligent. For us, the ability of a school of fish (as opposed to merely a shoal) to move as one amorphous being is quite amazing and a feat which requires a whole array of signals to be processed and acted on, yet these fish will almost certainly be doing it without conscious thought.
But, do we consider problem solving to be the best trait? The ability to respond to Pavlovian training?
And finally (on a lighter note), who can forget the deep sea anglerfish (of the Order Lophiiformes) where the male is parasitic and after finding a mate (who is many times larger than him) just simply latches on and receives sustenance from the female for life? Can't get much more intelligent than that in my book.