In the Lord of the Rings movies, Gollum is played by Andy Serkis who does an amazing job, despite the fact that he does not physically appear in the movie. Instead, CGI imagery appears in the final film, obscuring the physical form of the actor. However, all of his movements were captured by a motion capture system, and his voice is used, so in my opinion at least he acted the role of Gollum as much as, say, John Rhys-Davies acted the role of Gimli.
The thing that stood out the most to me when I watched the movie version of The Two Towers was how well the dichotomy between the nasty Gollum and the pitiful Sméagol (also known to Samwise Gamgee as Stinker and Slinker) was portrayed. In the novels this distinction was made, but it was very apparent in the movie, in a scene reminiscent of one in the Spiderman movie. I really felt sorry for poor Gollum in the movie, whereas in the books I tended to think of him more as an unpleasant creep whom Frodo and Sam had to put up with in order to make it into Mordor. In the movie, you genuinely got the sense that he was a hobbit who had possessed the ring for too long, and that Bilbo would have turned into something similar if he'd held the ring that long. Yet another example of Peter Jackson playing up the evilness of the ring.