Not a crenicichla, but you could do a single Erythrinus erythrinus (red fin wolffish/rainbow wolffish), Erythrinus sp peru (purple wolffish), or Erythrinus sp Tahuayo (white tip wolffish) being the most aggressive. These fish do not swim around much though. They just sit, waiting for food.
If you want constantly swimming fish, a school of 16+ exodons (Exodon paradoxus) aka bucktoothed tetra.