Quality is subjective. There are elements of films that add to enjoyment for most people, (i.e. good actors, good cinematography etc.) Personally, I want a movie that immerses me. If I forget that I am slumped on the couch and sloppily eating popcorn, then it has succeeded in its purpose. Some movies rise above those expectations by posing interesting philosophical questions, defying the norm, using versatile actors, possessing interesting dialogue, etc. Those movies, the ones that do more than simply immerse me by also intriguing me, those are the ones that have lasting appeal and cement themselves as great movies. However, many film-makers, in their attempt to embellish a story to make it more interesting, will remove the audience from the immersive world they created to remind them that they are only watching a movie. Take, for example, The Lady in the Water. Most would agree that the film is sluggish, insipid, and fails to have any significant message. The director, (Shyamalan) tries to establish motifs without subtlety (in one case by literally naming one of the characters "Story"), indulges in idiosyncratic dialogue, and gives every single character a quirk in favor of character development. Context is practically absent from most of the film, and when there is context, it is an inscrutable mess.
Qualities such as idiosyncratic dialogue or motifs, when used by other film-makers, can make a story more interesting. Pulp Fiction uses both to establish character (i.e. Vince Vega's vast knowledge of pop culture and social awkwardness) and leave the audience intrigued (i.e. the briefcase that *spoiler warning* is filled with something so magnificent, but is never seen by the audience.) The movie doesn't come off as boring or pretentious either because it still tells an interesting story regardless of whether the audience members are looking for motifs or interesting tidbits about a character and because it employs a Tarantino-brand mix of low-brow and high-brow humor which is still interesting for the lay moviegoer without sacrificing its core theme of redemption and "doing right" in a world gone wrong.
The fundamental difference between the two movies is fun. Shyamalan uses the elements of movies to create a story that is both boring as meaningless, while Tarantino uses them to tell a cohesive, philosophically potent story while not veering too far into high-brown nonsense and maintaining audience enjoyment. I enjoy watching Samuel L. Jackson and John Travolta joke about foot massages right before bursting into a room and assassinating their target. I don't enjoy whatever happens in any of the scenes from The Lady in the Water.
So yeah, an immersive story with intriguing elements are the most important things to me.
As far as my favorite movies:
Fight Club
Pulp Fiction
Mulholland Drive
Kingsman: Secret Service
Thank You for Smoking
The Great Debaters
Groundhog Day
Forrest Gump
National Lampoon's Christmas Vacation
There are more, but I can't think of them right now.
