Sauron's a little ♥♥♥♥♥, first off. You never see what he looks like in the movies IIRC, and he's killed when Frodo destroys the ring in Mount Doom. That's it. No epic fight, no final battle. Meh. It seems Sauron relies on the Witch King and the other Nazgul riders and trolls and orcs to get anything done, but if it's just one vs one vs one, what chance does he have?
Which leaves Voldemort and Ganondorf. Voldemort has strong dark magic, Ganon kind of does, but definitely not to the extent that Voldemort does. It's not even a fair fight, to be honest; Voldemort has/had the Elder Wand, seven horcruxes, and all kinds of strong-as-♥♥♥♥ spells (5th movie glass shard impale on Dumbledore?). Ganon has the Triforce of Power, but he's been beaten by Link, what, seven times? I like Ganon, but he's definitely the weaker of the two.
"What do you want from me?!"- Vanille, Final Fantasy 13.