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.

Anyone see The Shawshank Redemption? This movie is AWESOME.

I also like LOTR and the Matrix movies a lot

Ok, I'll bite.  My top 5 movies....hmmmmmm

I am going to pick movies that I have seen more than once, and would definately watch again.  They are not based on their sound track necessarily and not really in order.

1. Young Frankenstein

2. Wizard of Oz

2 1/2. Star Wars - All of them (I'm cheating here)

3. Willy Wonka and the Chocolate Factory (the first one)

4. Apocalypse Now

5. To Kill a Mockingbird