I would try:
- well of course Nodame ( but the romance is not really the main plot)
- Zettai Kareshi
- Proposal Daisakusen
- Love Shuffle
-Tatta Hitotsu no Koi
- Hana Kimi ( more comedy than romance, but it is a must-watch)
If you want some more serious heart-wrecking romance, you should really try Korean ones, but if you're into more cute romance you should check Taiwanese dramas.