Probably the scene where he gets Douglas into bed and she’s laughing hysterically. It’s so intense with her happily laughing while we the audience know something horrifying is coming. It’s perfect how Scorsese drags it out a bit and has her laughing the whole time before it to buildup the tension.
Kiss my black ass! Toodles!

For me, hands down, the scariest moment is when Max Cady hears Sam Bowden clumsily make a noise behind the dumpster.
Can you even imagine trying to stay still, hearing that lunatic approaching, wondering if he's going to stop, knowing what will happen if he finds you back there?
"Counselor??? Is that you? Could you be there?"
I would have peed my pants.

Yeah the sudden reveal of Cady dressed like the maid creeped me out the most.

Omg yes agreed on the end credits. After the amblin logo and just hearing the rumbles of thunder and those birds.freaky. I keep telling myself we will hear cady's voice in the distance. Shivers.

De Niro dressed as the maid killing Joe Don Baker

All the scenes you mentioned are creepy. Also, this is one movie that has the creepiest end credits. Just listen to those sounds toward the end!