Some Like It Hot - Tony Curtis and Jack Lemmon are a brilliant combination in any situation but this one probably wins out for me. The fact that the AFI considers it the best American comedy of all time speaks for itself.
Die Hard - It's simple and formulaic but it works. The second one was nowhere near as good, the third one came close thanks to Jeremy Iron, the fourth was was a bit naff, and I haven't even seen the fifth one. However, the performances of Willis and Rickman in the original are just perfect, my favourite performance by each of them.
Trainspotting - I like most of Ewan McGregor's films but this one and Shallow Grave stand out for me. Not only does he provide a stellar performance in both, the rest of each main cast match it, making both films great. It's a close call between the two but I think Trainspotting edges the victory because its surprisingly slightly less surreal.
The Three Musketeers - Although I like all of the film incarnations of it (even the one with flying ships), the 1970s version with Michael York, Oliver Reed, and Charlton Heston is far and away the best. The story stays closer to the original story, which I'm currently reading, than either of the more modern versions, and also manages to inject its own unique spin on it through the comedy used across the trilogy.