Baryonyx are quick like Allosaurus and do just as much damage (but don't get a pack bonus). They don't use oxygen for swimming and can sprint in the water. Right click in the water does an AoE stun that lasts about 10 sec. Their 'C' is a... Taunt? They also jump and you can fire a weapon from their back.
I would argue that this dinosaur is the strongest mount that can fight on both land and in the water. I had one that was bred multiple generations, full imprint, purple saddle. I named him Fury to represent all of the anguish I experienced leading up to his creation. He was able to handle anything I threw at him with ease. People were surprised to hear that I was easily able to kill both squids and alpha carnos with him. Never got to attempt an alpha Rex. He met his match when I attempted an alpha squid. RIP Fury.