[Kalimac]

I'm pretty sure it's Sauron, mostly from the passage that Diamond already cited. (Especially since in the book Gandalf speaks of the Ring wanting to return to its master - if the Ring is the Lord of the Rings, then what on earth would its master be?)

"The Lord of the Rings" is actually a really good all-encompassing title, because that's where the germ of the story is. Think of Daphne du Maurier's "Rebecca" (Last night I dreamt I went to Lorien again...oh, wait). The title character is someone who is dead before the book opens, but the only reason the story exists is because of things that she did and events that she set in motion; she's the kernel of the plot. Same with the Lord of the Rings; everything that happens in the story happens because he existed and because he made the Ring. Every single event. What other title could have included everything so thoroughly?