The Fellowship of the Ring
Sauron is the most obvious antagonist here. First, he's an enemy of Frodo, our protagonist: Frodo is trying to destroy the One Ring into which Sauron placed some of his power many ages ago. On top of that, Sauron is also an enemy of, well, all of Middle-earth. Sauron's goal is to cover the whole world in darkness, under his sole dominion. This guy is an Enemy – with a capital E – and he sticks around for the whole Lord of the Rings series.