Mollies. I got them, then they bred, now I need them gone. I also regretted getting my various goldfish, but those have since been rehomed, so all ends well, I guess. Oh, and corys. I just have no luck with them, no matter how much I try to get them to live they don't make it. I haven't had any in a while now, don't want to kill any more of the cute little fellas. Oh, and a single, completely evil convict I once had. Every time I would walk past the tank it would try to bite me through the glass and several feet of air, and I got a little scared of reeding it, for fear of losing fingers (not that it could have taken any off, too small, but definitely evil enough to try...) It jumped from the tank and died, and I was sad, but somewhat relieved too. Any and all of the wild fish I managed to catch when I was a kid, any and all oversized fish I ever found/got, and any and all overly aggressive fish I ever got.
And those are the fish I regret getting.