Ok, off the top of my head...
Star Wars Trilogy (original)
Although GL seems to be ****izing our beloved childhood favorites on the eagerly anticipated DVD release, and nearly sinking the enterprise on the pre-equals (that are not worthy to carry the SW seals of the originals) what these films did for the film industry hasn't been replicated since. THX, Skywalker sound, etc. were created from these movies alone. ANH was a pioneering acheivment, and ESB was the first cliff-hanger of it's magnitude.
Casablanca
I only watched this movie for the first time this past year, but damn did I fall in love with it!
Gladiator
I was blown away the first time I saw this moive!
Lord of the Rings trilogy (Fellowship of the Ring)
Again, setting the visual standard for movies to follow.
Halloween
I generally do not enjoy horror movies, but I love this one. Maybe it's the idea that it could really happen and that is what makes it so good? I think it finds itself on this list because this is a genre I normally despise, and somehow this movie overcomes that and for that, it is highly regarded IMO.
I know I am leaving out some great classics (and probably many I have never even seen!). I'm sure I'll think of 10 other movies I could put in my top 5 as well, but off the top of my head, these are it! What are yours???