All the responses so far have merit. I think the respondents must all be young, however, because, favoring a single species or even a single family of fishes, for me, becomes increasingly difficult with the passage of time.
I have always had an ongoing attraction to Marine Angels and Butterflies. They carry a connotation of exotic places, beautiful reefs, the mystery and secrecy of the details of their natural history and their frequently audacious and extreme beauty. To interact on a Caribbean reef with a French Angel is a magical experience. To observe the extreme delicacy in pattern, anatomy and physiology of the Chaetodon Capistratus is a wonder. These creatures live in such harmony with what surrounds them in their native waters. I am indeed lucky that the ones I own humor me to the point where they agree to leave the Indo Pacific to be contained in a few hundred litres of water in a Midwestern living room.
But I see where the rest of you are coming from, truly I do: I have kept oscars among many other large Cichlids, and they are intelligent, attractive rewarding fish. And I was long obsessed with bettas in the seventies when the "veiltail" was the NKOTB. I am perennially entranced by the serene dignity of
Koi as they move silently though a garden pool like maiden-princesses enroute to an unseen tea ceremony. And the Aulonacara Jacobfreibergii, in any of its many morphs, with its elegant fins and tail that earned it the name "Malawi Butterfly" is to me, the pinnacle of peacock beauty.
Love whatever you want. Love it well!
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I love how big and beautiful they get. They are so majestic swimming in a huge pond. Ponds are beautiful, but large, colorful 
