I'd say Kit. Her entrance was the catalyst for a dramatic and tragic occurence. Nicole on the other hand was more I'm here for what's mine, c/w a Chase backstory.
Kim Novak´s first appearance was very special to the fans of classical movies: she had been retired for a long time, and was a part of many memorable films (not only "Vertigo"..). She didn´t need any award recognition or showy roles because she just wasn´t than kind of actress. It was sort of a reunion with someone you had good times with in the past (not that kind, at least not for me...) so the whole package of Kim on FC was a big gift to her fans, beyond her character´s meaning in the plots or its undoubtedly but irresistible fan-ficcy quality..
Leslie Caron had been more active in European movies mainly. She had Oscar nominations and also was in many memorable movies. She gravitated towards more intellectual, subtle performances. But her storyline here was absolutely contrived, and seemed to be made to tarnish Chase´s memory to the max, at least in the beginning. I´d dare to say that Caron had zero idea of how to play her, Nicole was just 100% bitchiness on high heels, and her wardrobe was horrendous. Only her entrance was very enjoyable, though a huge cliché. Like "Falcon´s" own Lady Ashley...
You can rewatch both entrances in one of my latest videos. It´s about the first lines, so in Nicole´s case, it´s about when her face was first seen and her first lines spoken:
Notes: Go to 11:30 to see Kim´s entrance, with a blurry, cheap photography that doesn´t do justice to her (not does her wig either).
Go to 12:35 to see Leslie´s entrance, pretending to be on a bridge in Paris (she wasn´t, she was in L.A. I think).