[Gothmog]

I think it's wrong to decide how powerful a being is by stuying the battles he/she/it might have won. Just look at the fight between Fingolfin and Morgoth. Fingolfin challenged Morgoth and scared him. Morgoth didn't want to leave his safe halls of Angband, but was forced to. Then the battle began and Fingolfin attacked Morgoth time after time. Seven wounds he inflicted and Morgoth cried out every time. Then, being a bit tired, Fingolfin stumbled and was then crushed. But what if he hadn't stumbled? Does the fact that Fingolfin was equal to a Vala, the greatest of them, in battle imply that he was the equally powerful as Melkor? I think not.

So even if the outcome of a fight between for example Sauron and a Balrog could be predicted, that tells us nothing of how powerful either creature is. That depends on the meaning we put into the word "powerful".

If you want my opinion, I'd say Balrogs is greater in might, on the battlefield. They're one of the most terrifying creatures of Morgoth. But Sauron is more powerful in other ways. A Balrog is not fit to command armies, nor would thay have the power to snare men in nets of treachery or make powerful items like the One. Also, I think of Sauron as a great master of witchcraft. So to decide who's most powerful or mighty, we must decide what characteristics we're looking for.