Before Sunrise
I feel like this is one of those films where you don't realise how invested in it you are until you reach the morning with them and absolutely feel the dread of their time being almost up looming over you.
Just an incredible bittersweet ending, too.
I'm kinda glad I found this film late, as waiting a decade to find out what happened next (in what has got to be the best sequel of all time) would have been hell to wait through.