There are certain relatively common fish I think are ugly like:
Red Devils/Midis
Goldfish (especially them ones that look like their brain is sticking out)
Bushy Nose Pleco (their face weirds me out)
Serpae Tetras
Any Glass(see through) Fish (other than Ghost Cats)
Flounder (not really an aquarium fish but they are ugly, but taste delicious)
Blood Parrots (why were they even created, were they trying to make it look like a goldfish?)