Before Midnight
Everything about this one catches you off guard, the first part is disorientating as it's a completely different style to what came before, then it gives you what you came for and you get a bit more comfortable with, but still feel like something is off, then it just kicks you in the nuts for the rest of the runtime.
It feels a lot more open to interpretation, too. I think the ending is very sinister but a lot of people see it as a happy one.
Talking of endings, that's something I feel like Linklater and co absolutely deserve more credit for on this trilogy:
Sunrise: fantastic ending, but I NEED to see what happened next.
Sunset: perfect ending, but I NEED to see what happens next.
Midnight: sure, I'd love to revisit these characters again, but I have absolutely no need to, it feels complete.
Having a fourth film would run the risk of having an unsatisfying end.
Ethan Hawke & Julie Delpy are fucking sensational throughout and whilst there's a wealth of incredibly talented people out there, I don't think any other combination of actors could have ever carried these films in the way they did.