Ooh, people already mentioned some movies I really love. I'd add Trainspotting, and also Shallow Grave, which is a movie by the same director, Danny Boyle. Actually, pretty much everything he's done is really good, and has some existential elements. But Shallow Grave is one of my favorites, it deals with how a group of friends react when they find a dead guy with a suitcase full of money. As you can probably guess, it's a drama with some darkly comedic elements.