18. Gollum - Andy Serkis
This first entry creates a paradox. If the LOTR films hadn't come out before, then chances are, Andy Serkis wouldn't be as well known as he is today. Yet it's hard to see anyone else being cast in this role under any circumstances.
Serkis is the godfather of motion capture, thanks in no small part to LOTR, and there's no one more qualified to scamper around as everyone's favourite ring obsessed, dental nightmare, Gollum.
Which makes this seem like a weird, alternate timeline entry, where Serkis would end up doing his Caesar performance first, then Gollum. What a headache this is turning out to be.
As Gollum was one of the few characters that was pretty much exactly like his counterpart in the books, there's not really any need to put a new spin on the character, and any attempt would likely suffer under direct comparison. The novelty of the character can come from watching Gollum interact with the new Sam and Frodo. Even the twenty year gap between performances shouldn't matter, thanks to the nature of the role.
With brand spanking new technology, Serkis can appear as fresh as he did back then. Ah, if only we could all run around as ageless CGI mo-cap people. It'd solve a lot of the world's problems.