In no order:

1) Rear Window - I watch TCM more than any other channel.  I love what a good director can do in Black and White.  I also like the fact that the old movies rely on acting to carry the story.

2) Gladiator - Possibly the best epic of all time.

3) Open Range - Have to have a western in there and I just like this one for some reason.

4) Forbidden Planet/The Day the Earth Stood Still - I can't decide

5) Any movie that changes or creates a new genre.  Star Wars IV, Raiders of the Lost Ark, The Matrix, Kill Bill and many others

Can't say that I have a permanent Top 5 since the list changes constantly:

Alien(s) - My favorite of the 1st two depends on which one I watched last

The Apartment - Classified as a comedy but in my opinion it is a drama with some pretty funny lines

Caddyshack - Can be classified as nothing but a comedy and my favorite one. 

Casablanca - I avoided this movie for years.  All I had ever seen was clips from the final scene at the airport with Bogart and Bergman.  Nothing wrong with that scene, but I thought the whole movie would be like that.  When I finally saw it I understood why it is a classic.  I think Claude Rains nearly stole that movie.

I have to move beyond the first 3 letters of the alphabet so the final movie is:

Lord of the Rings - They are really one movie split into three parts so I will count them as one.