Depends if you class large catfish as oddballs or not, if you do then Mystus nemurus the asian redtailed catfish is widely thought of as the most aggressive fish in the world, in the wild they have been known to attack and injure humans and will happily take on fish many times larger than themselves and eat them a piece at a time!
If not then it would be a toss up between The fahaka puffer ( Tetradon fahaka ) or the red snakehead ( Channa micropeltes ). Both these fish get to sizes in excess of 30" and will attack and eat anyfish that come into contact with them.
Other contenders for the title would be the wolf fish ( Hoplias malabaricus ), black pirahna ( Serrasalmus rhombeus ) and the sabre toothed tetra ( Hydrolycus scomberoides ) all of which are pretty nasty customers too.