For me, its Don Penguini (Legendary, non-ultra [Almost able to ultra evolve, 80% currently])
I had him as my only monster in my team for 3 of the Grand Four, and The Grand Champion match and it WON ALL OF THEM ALONE (I had a few defeats though). Being the only monster, it was able to use Last Bite, which deals a massive amount of damage (and only took 50 sec), not only that it converts 40% of the damage dealt into hp for itself.
Penguini and dragaia are both pretty op on their own.
I would say either rexo, because blood fury kills everything in 1 hit, or perhaps aegis dragon just for sheer utility.
Bahamuzar is a possibility but it needs support to work.
Not being an owner of Rexo / Last biters, I have to say Ultimadragon + Seraph. Between them they sweep teams (double survivor about 4 guys, then bloodthirst, with seraph risky healing)