I'm looking at this question semi-scientifically.
When you look at the 10 "best" of anything, you have to look at the characteristics of that "thing" and then find examples where every identified characteristic is exemplary. For movies, I think the key characteristics to look at are:
- Story/Plot
- Acting
- Directing
- Music
- Cinematography
The following films are absolutely top notch in all of the above:
- Jaws
- Citizen Kane
- The Shawshank Redemption
- Chinatown
- Casablanca
- The Godfather
- Die Hard
- North by Northwest
- Alien
- Lawrence of Arabia
Are they the best movies of all time? I'd say yes, and the reason I didn't swap out other films (such as Star Wars or The Dark Knight) was they had slight deficiencies compared to films I included (The Dark Knight had some plot problems, for example, and Star Wars has some acting that is not quite top shelf)