5 - Christopher Walken. I know, I know, it's Walken and therefore I should have seen everything he's in, but I haven't seen a single movie where he's a lead role. I've seen plenty of him on tv shows, but again in smaller roles, and I saw him host SNL once. He's a really cool character and if I see more of his movies it would help me finally compile a top 5 actors list.
4 - Al Pacino. Surely there's more movies of Pacino where he's just another tough guy that kills lots of people. I've seen Scarface, the Godfather Series, Scent of a Woman, and Insomnia, Devil's Advocate. Of course, it's Pacino, so there's about a billion other movies he's been in, but with so many choices it's hard to finally pick some without feedback.
3 - Kevin Spacey. I agree with many other people here by saying Spacey is inarguably one of the greatest actors known today, but I don't know very many films he's been in. Of course there's the Usual Suspects (loved it), American Beauty (loved it), and I fell asleep during K-Pax.
2 - Ewan MacGregor. Although many people I know love him for his role in Trainspotting, I love him for playing Grimes in Black Hawk Down. It's like a character in Office Space or Clerks' Dante got stuck in the Somolian conflict. I was just waiting for him to yell "I'm not even supposed to be here today!" or "I swear if I keep missing these people shooting at me I'll just throw my M-16 out the window!" Besides the crappy Star Wars movies he's been in, I've only seen him in the aforementioned movies, and that's it.
1 - Ethan Hawke. A few months ago people were non-stop raving about how great Sunrise/Sunset are, but the only two movies I've seen Ethan Hawke in are Reality Bites (which I hated) and Training Day (loved it). If you were to ask me what other movies he is in, I could only answer "Hamlet", because I really know nothing about this guy.