I believe that Tolkien several times calls Melkor/Morgoth the most powerful being created by Eru, the most powerful of the Ainur, the most powerful of the Valar, etc.

Therefore the most powerful possible pair of combatants would be Eru and Melkor/ Morgoth.

So I suggest that the most powerful battle would be:

1. Melkor/Morgoth vs Eru, if the whole of the Elder Days is considered to be Morgoth's rebellion and figurative "battle" against Eru.

2. Morgoth vs Tulkas, as in Edlothiad's answer, if it has to be a literal battle and if Tulkas is more powerful than Ungoliant.

3. Morgoth vs Ungoliant, as in Daishozen's answer, if it has to be a literal battle and if Ungoliant is more powerful than Tulkas.

I believe that Tolkien several times calls Melkor/Morgoth the most powerful being created by Eru, the most powerful of the Ainur, the most powerful of the Valar, etc.

Therefore the most powerful possible pair of combatants would be Eru and Melkor/ Morgoth.

So I suggest that the most powerful battle would be:

1. Melkor/Morgoth vs Eru, if the whole of the Elder Days is considered to be Morgoth's rebellion and figurative "battle" against Eru.

Or:

2. Morgoth vs Tulkas, as in Edlothiad's answer, if it has to be a literal battle and if Tulkas is more powerful than Ungoliant.

Or:

3. Morgoth vs Ungoliant, as in Daishozen's answer, if it has to be a literal battle and if Ungoliant is more powerful than Tulkas.