The most aggressive cichlids I would suggest for a 50 would be a pair of large convects. The key words here are the most aggressive, you'll get some off the wall suggestions but you wouln't find a more agggressive cichlid suitable for a 50 gallon thank than a convect.
Hi, welcome to mfk! If you want something colorful, active and super aggressive for a 50, try Salvini. After a long time keeping lots of aggressive fish I just got my first ones and they're crazy. Mine are small but raise hell with fish several times their size. Salvini!
Pound for pound i would say go with the "mini Dovii" the Grammode,these fish are killers and i believe that they can live in a 55gal tank.Plus they look just like a Dovii.