Sauron was Morgoth's greatest servant. It can be disputed that Gothmog Lord of Balrogs was an equal to him. If you mean which was better in terms of a fight I would probably say a Balrog but if you mean which was more powerful in terms of 'higher in rank' of the Ainur then Sauron would surpass a Balrog, with the possible exception of Gothmog.
In Tolkien's earlier writings Balrogs had much less power, but compensated this in numbers. In time however their number reduced to a final sum of 3-7, on the other hand their power definitely increased. About your question, Sauron is the greatest servant of Melkor, and second in command, however it is said that Gothmog had hardly less power then Sauron, so the differences weren't that big.
I wonder if Gothmog was equal to Sauron or inferior? I seem to recollect a quote naming them equals but then seeing others suggesting Sauron was superior.
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Utúlie'n aurë! Aiya Eldalië ar Atanatári, utúlie'n aurë! Auta i lómë! Aurë entuluva!
Give me a Balrog any day. Sauron is a deciever. Balrogs are more fighters and are not as political as Sauron. They are cunning in a different but predictable way.
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I would rather face Sauron in battle than a Balrog though. At least there is something to hit. With a Balrog the outer layer is just flame and shdow. It makes you wonder how Glorfindel managed to get so close to the Balrogs body to stab him with his Helmet.
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Utúlie'n aurë! Aiya Eldalië ar Atanatári, utúlie'n aurë! Auta i lómë! Aurë entuluva!
