Here is my current Top 5. This is without including Ayumi. I'm mostly interested in JPop from the 80/90s. I could easily mention more than 20 artists.
1. 松田聖子 (Seiko Matsuda)
2. 中森明菜 (Akina Nakamori)
3. 宇多ヒカル (Hikaru Utada)
4. 酒井法子 (Noriko Sakai)
5. ZARD