May was the first big one that lasts to this day. Misty was ok and I liked her prettier moments, but I was never overly attracted to her. And we got some good looking filler girls in eps, but given they're only in a few eps, there wasn't much to say.
When May first debuted I was like, "Wow, and we're going to see this girl in every episode now?" Her boobs were bigger than some of the adult characters in the show, and I loved her outfit and girlier personality. Of course being a teen when Hoenn came out helped.