My ACF (not really a fish I guess) has to be the dumbest thing in my tank. He can smell food, and will swim around scooping water towards his face, but he'll come to rest sitting on top of a chunk of meat without touching it. He will gobble anything that he actually catches by accident, but he's completely useless at finding it.
My goldfish "Buddy" isn't too bright either. He tries to snuggle my oscar, and loses a scale or two, then goes to snuggle my bronze knife, and gets chased around for half an hour... then goes back to the oscar for love. *shake*
I wouldn't say any of the fish in my tank are overly bright

Although, if my oscar wants something, he will find a way to get at it, even if it means rearranging the tank. I'm not sure what kind of oscar isn't an agressive feeder... I have to watch my fingers when I feed mine, and he's well fed!
Meanest I've got has to be the bronze knife... I call him simply "angry". In the week I've had him, he's decided he didn't like my big moss ball, and has torn a bunch of tufts out of it. He's uprooted some of my more fragile plants, and he'll square off against anyone who gets close to him. Oh and he likes chasing Buddy around
