Not sure how small you prefer but here are a few of my favorite mini monsters.
Ctenopoma acutirastre
Nandus nandus
Badis
Ery ery wolf fish
They can live in a 55 gallon tank and possibly a little smaller.
Golden wonder would be fine in 36" tank. It is a monster to behold. Loves chowing down on tankmates half its size. It likes floating plants, too and is a topdweller.
IMO mini arowannas. They grow fast, so be careful with tankmates!