I love watching films! So I have seen a lot, many of which are old classics. In general I like comedies and action, but can watch almost anything - even girlie romantic cryfests. My favourites are usually those that "plays" with areas such as time. You can see my complete vote history on IMDb, but here is a shorter list of what I think are the best.
12 Angry Men [drama, usa, 1957, black and white]
300 [action, usa, 2006]
Adams Æbler "Adam's Apples" [comedy, denmark, 2005]
Airplane! [comedy, usa, 1980]
Alien [action, usa, 1979]
Blinkende Lygter "Flickering Lights" [comedy, denmark, 2000]
The Bourne Trilogy [action, usa, 2002/2004/2007]
Children of Men [action, usa, 2006]
Cidade de Deus [City of God] [drama, brazil, 2002]
Cloverfield [action, usa, 2008]
Crank [action, usa, 2006]
The Dark Knight [action, usa, 2008]
District 9 [action, south africa, 2009]
Donnie Darko [drama, usa, 2001]
Eternal Sunshine of the Spotless Mind [drama, usa, 2006]
Festen "The Celebration" [dogme, denmark, 1998]
Fight Club [action, usa, 1999]
Flushed Away [comedy, usa, 2006, 3D animation]
The Hangover [comedy, usa, 2009]
Hoodwinked! [comedy, usa, 2006, 3D animation]
Hot Fuzz [comedy, uk, 2007]
Inception [action, usa, 2010]
The Incredibles [comedy, usa, 2004, 3D animation]
Kick-Ass [action/comedy, usa, 2010]
Le Fabuleux Destin d'Amélie Poulain "Amélie" [comedy/romance, france, 2001]
Life of Brian [comedy, uk, 1979]
The Lion King [comedy, usa, 1994, animation]
The Lord of the Rings Trilogy [adventure, usa, 2001/2002/2003]
Martha [comedy, denmark, 1967]
The Matrix [action, usa, 1999]
Memento [action, usa, 2000]
Monty Python and the Holy Grail [comedy, uk, 1975]
Napoleon Dynamite [comedy, usa, 2004]
Pirates of the Caribbean: The Curse of the Black Pearl [adventure, usa, 2003]
Pulp Fiction [action, usa, 1994]
Ratatouille [comedy, usa, 2007, 3D animation]

Scott Pilgrim vs. The World [action/comedy, usa, 2010]
Se7en [crime, usa, 1995]
Serenity [action/comedy, usa, 2005]
Shaun of the Dead [comedy, uk, 2004]
The Shawshank Redemption [drama, usa, 1994]
Sin City [action, usa, 2005]
South Park: Bigger Longer & Uncut [comedy, usa, 1999, animation]
Spider-Man [action, usa, 2002]
Star Wars IV/V/VI [adventure, usa, 1977/1980/1983]
Underworld [action, usa, 2003]
WALL·E [comedy, usa, 2008, 3D animation]I will create an article in the future about which films that in my opinion are "must-see"-films.