It depends - by toughest do you mean the most powerful? If that is the case, then I've already posted my answers to this on another thread. But if "tough" means simple tougness, no bending involved, then I have to put together a rather different list to other people:
Air: Does Appa count? Because if he does, then Appa wins the raw body strength and endurance contest.
Water: Quite frankly, Hakkoda is the only Water Tribe person we've seen so far who goes in for the non-bending type of "toughness".
Earth: The Big Bad Hippo - this guy can simply stand there unmoving and let giant boulders smash apart as they hit him - not even Toph and Bumi can do that.
Fire: Combustion Man. As well as his great height and buffness, in WAT when Zuko tried to kick him, CM didn't even seem to notice - the kicks just bounced off. Maddokos - I was sure you'd be with me on this one
Edited on 08/04/2008 4:25pm
Edited 3 total times.