Top 50!? I don't think I even own that many DVDs...

The standard no particular order disclaimer applies:

Back to the Future trilogy (on TV constantly, but they're just fun to watch)
Batman
Dead Poets Society
Desperado
High Fidelity
Indiana Jones trilogy
LA Confidential
Lord of the Rings
Memento
Office Space
The Princess Bride
Requiem for a Dream
Shaun of the Dead
Signs (Sixth Sense is great too, but for some reason I enjoy Signs more)
Stand By Me
Star Wars classic trilogy
T2
Tombstone
Young Frankenstein
The Matrix

Other favorites I don't own, but should:
The Godfather 1 & 2
Any Tim Burton film (except Planet of the Apes)
Charlie and the Chocolate Factory (Gene Wilder version)
The Boondock Saints
Pulp Fiction
Clue