: Surely no other character comes close.

: TreeBeard..................

I disagree, otherwise, why would Elrond make the statement that if the Ring were not destroyed, Bombadil too would fall before the might of Sauron (albeit "Last as he was First")?
This would imply (to me anyway) that Sauron (as the most powerful of all the Mair) was more powerful. The Istari could have been either more or less powerful in thier original forms, but being "incarnated" were probably weaker. It seems to be a recurring theme that turning to evil reduces ones power (a la Morgoth), so the Balrog may have been less powerful as well.
I guess this would all depend on who or what Bombadil actually was. If a Mai, he would be weaker than Sauron, if a Vala, stronger, if a manifestation of the earth or nature, it is very debatable.