1. Taxi Driver
My personal favorite of his films and also my favorite film of all time. Taxi Driver is a character study that can stand with the best the genre has to offer while also presenting one of the greatest characters in cinematic history. Robert De Niro's Travis Bickle is a deeply flawed man who just can't seem to interact properly with the outside world. The King of Comedy is an extension of this type of character while trading the brutality of Taxi Driver for awkwardness and dark humor. Travis Bickle, despite being a murderer, is a man that we can all relate to because he is a lonely person just trying to find someone that he can connect with. Scorsese takes this universal theme and uses it to present a very complicated character that is morally ambiguous. The ambiguity is something that keeps Taxi Driver endlessly re-watchable since there is always something new to see in Travis and his motives for becoming a vigilante.
Thanks so much for reading and I'd love to hear about your favorite Scorsese films in the comments.