Hmm. I have no definitive "best", but I have strong favorites per category/niche-- some of which I'll mention below:
- Strong, smart female leads. She doesn't have to be a rocket scientist, but she has to be at least sensible: Saiunkoku Monogatari, Okobore Hime to Entaku no Kishi, and Song of the Long March .
- The ones that skewer the shoujo romance genre lovingly: Ouran Koukou Host Club, Koi dano Ai dano, and of course, Gekkan Shoujo Nozaki-kun
- Historical/political romances, fantasy or real life-based: Reimei no Arcana, Akatsuki no Yona, and Versailles no Bara.
- Best-friends-turned-to-lovers plot, which I listed here
http://syaochan.com/blog/archive/8-sweetest-best-friends-lovers-romances-anime-manga