Easily freshwater rays. When I first got into the hobby I have always wanted them. I got my first one two years ago and havnt looked back since.
I cant pick one species but leos, marbles, and most hybrids (coincedentally, my 3) are hard to beat.
Mines gotta be jardini and has been for quite a few years now , although I'm a big fan of big cichlids they were my first love when it comes to monsters