If they fought in water, the kraken.
If they fought on land, the roc or the mokele-mbembe.
If they fought in the air, the roc.
If they fought in space, they all lose.
If they fought in space in a huge air tight spacestation/biodom with no gravity, the mokele-mbembe, but then he would eventually get lonely and take his own life so...
Why are they all fighting in the first place...?