I was just thinking, which is the best demon in the game? Is it a Baalor, Pit Fiend or just the simple Cacofiend.

That depends on your definition of better, I guess. A balor is worth 26000XP, which is more than a pit fiend's 9000XP. A marilith is worth 23000XP. A nabassu is worth 16000XP, I think. Balor's can cast hold person, which can be irritating, and according to the books by Salvatore, they're the nastiest demon types around, and the rarest.

well, i mean those of the demon kin you can summon, which is the best?

"Gate" will call a pit fiend, so i bet this is the best of them.

I've never bothered that much with demon summoning spells as the summons just never seem to last that long. Compared to a Deva, a pit fiend just isn't worth the trouble.

The Balor is the Tanar'ri equivalent of the Baatezu Pit Fiend, and both are the most powerful of their respective races IIRC. So they should be about the same in power.