Scenes that could be self contained as their own short films or that got you hyped for the rest of the movie etc you get the idea. The first two that come to mind for me are Scream (the first one in particular) which has just an iconic first scene that imo might be Wes Cravens finest piece of work (hard to say though)
Inglorious Basterds also has an incredible first scene.
The Dark Knight also comes to mind but I’ll stop, I wanna hear your favorites.