Cool thread idea!
Of those that I've seen, most of which multiple times:
1. Raging Bull - Sometimes, it's the simplest stories that have the most impact. This is little more than the life of a mindless brute who manages to destroy everything and everyone around him. But everything about this film oozes class and purpose. With all the talk about toxic masculinity these days, this film knows how to humanize this sort of character without ever making any excuses for him. Probably my favorite De Niro performance overall.
2. GoodFellas - Film history. Equal parts terrifying and fascinating. Endlessly quotable. Pesci deserves all the awards in the world. Better than
The Godfather. I said what I said.
3. The Departed - I've seen
Infernal Affairs and I love
Infernal Affairs. Nevertheless, I genuinely feel like this film is one of the very few examples of a Hollywood remake surpassing the original (won't hear me say that every day!). As far as police thrillers go, it's up there with the best of the best. Perfect cast, unforgettable writing. Vera Farmiga's character being poorly utilized is the only issue I have, really.
4. Taxi Driver - An impeccable character study, and as relevant today as it was 43 years ago. Plus Harvey Keitel is

.
5. Casino - An over-indulgent retread of
GoodFellas filled to the brink with despicable characters and barely held together by its gratuitous violence and expensive set pieces. And yet I love every single minute of it. Sharon Stone is
brilliant in this, doing the absolute best she can with the rather superficial role she's been given. And that desert scene! Most filmmakers go their entire career without ever attaining that kind of class.
6. Gangs of New York - Day-Lewis delivers one of the greatest (albeit scenery-chewing) performances of the century. Plus a surprisingly underused setting among historical epics.
7. Silence - Fascinating subject matter and impeccable cinematography. Top tier acting. Not exactly an easily digestible crowd-pleaser, but a must-watch regardless. You can tell how much Scorsese cares about the material he's dealing with here.
8. The Aviator - Very interesting film although a bit too meandering for my tastes (like most biopics). Cate Blanchett might be my favorite Hollywood actress working today and she is
mag-ni-fi-cent in this.
9. The Wolf of Wall Street - Difficult to watch at times, but gets its point across very effectively.
10. The King of Comedy - Difficult to watch at times, but gets its point across very effectively.
11. The Irishman - Perfectly watchable with a handful of great scenes. Would have benefited from tighter editing and less CGI jank.
12. Shutter Island - Nice soundtrack and interesting story beats. But Scorsese's directing style seems like an odd fit for this sort of film. Maybe Fincher could have traded this one in exchange for
Benjamin Button and everybody would have been happy.
13. Mean Streets - I get that it's his first one and therefore historically significant and all, but it didn't do much for me. I might need to watch it again.
14. Cape Fear - Much has been said about De Niro's performance in this, and for good reason. But this type of story greatly benefits from having an unshakably heroic protagonist, and Nick Nolte doesn't come anywhere near close to possessing the same sense of moral gravitas as Gregory Peck (but then again, who does?). I also prefer the original's more sober directing and cinematography.
15. The Color of Money - Some nice scenes. Paul Newman is always watchable. Rather unnecessary overall.
16. Hugo - Not bad, I'm just not the target audience.