For the past 7 years I have made a habit of watching Casino at least once a month, and tend to re-watch the following scene a couple of times:
https://www.youtube.com/watch?v=gcAaILQ0ATo
I just find De Niro's intimidating presence in this scene very believable and quite scary.
Also I tend to watch the following movies the most:
The Godfather (Part I & II)
Back To The Future (Trilogy) - I think throughout my entire life I have watched the second film easily over 200 times
The Princess Bride
Batman (1989)
Teenage Mutant Ninja Turtle
2001: Space Odyssey
T2: Judgement Day
Waynes World
It's funny that while going through the thread I noticed some have mentioned both TPB and TMNT, and I wonder how we share the same movies. But I think for me, its because overall its just nostalgia.
The following scenes are also frequently watched just for the amazing tone/execution of the scene:
https://www.youtube.com/watch?v=VqLETO_epi8
https://www.youtube.com/watch?v=HwBmPiOmEGQ
https://www.youtube.com/watch?v=em7EcaXPJF8
https://www.youtube.com/watch?v=TkyLnWm1iCs
https://www.youtube.com/watch?v=ITS5GkkqgEQ
Out of my 1000 plus collection of movies this is what all sticks out, but I could definitely put together a longer list. And for those who aren't aware I am considered a movie buff by those who know me best.