I would say hybrids because I loath them, but since they are not actually a species they do not count IMO. Many have listed Oscars but I like them. Don`t currently have any but still like em. I would not look down on a species because of it`s popularity. Fish come and go from favor. Way back Cubans were in (the hobby) and then disappeared completely, had to be re-imported.
I guess if the question was rephrased to what will you most likely never have time or space for I`d say rostratum. It just does`nt do anything for me.