Angels vs. Demons and Devils.

I disagree, though metamagic feats will make the Solar more effecetive. 4+ level 9 Divine Spells would let the Solar keep up easily, unless the DM is a fool. Summon Monster IX is pretty good, and lets the Solar occupy the Balor with big gribblies while the Solar casts Forbiddance. Then it's game over for the Balor.

As a party member, I would infinitely prefer to fight a Balor (plus friends) than a Solar, or any other level 20 Cleric.

Good > Evil in almost any fantasy setting.