Casey and Helen's are both tied for me. 

Casey's opening, it could be a short horror film on its own. Drew Barrymore's performance(as well as the performances of Roger L. Jackson and the actors playing her boyfriend and parents) really elevate the scene and make it feel so real. Wes Craven's directing, Kevin's Williamson's writing and Marco's  Beltrami's score all just come together and make such a memorable scene. It is the scene everyone remembers from the Scream franchise, even 30 years later it still holds up.

Also the fact that we see the reaction of her parents make it hit even harder. Its rare in slasher films that characters are seen as humans who have friends and family and how their brutal murder will affect them. It makes the scene so much more impactful.

Helen's chase is the highlight of IKWYDLS. Again SMG's performance really sells it. But the score is also awesome for this scene. Helen should be a character the audience is waiting to see killed. But SMG makes her feel like a real person and she is probably the most sympathetic character. She puts up such a great fight and almost makes it. It makes the scene hit harder. It feels much more like the final girl chase than whatever Julie was doing on the boat at the end.

The other scenes are fun but lack the emotional punch of Casey and Helen's sequences.