Great male supporting characters that will give you second lead syndrome. 

Most caring, devoted, loyal, and handsome ML who are insanely in love with FL (kinda obsessed). All of them are my favorite ones. 

Shows where the leads are adults already working somewhere

Good dramas where ML and FL slept with each other at some point in the story

ML is much richer than FL

Cute/ righteous / good-boy type ML in Chinese dramas.

Dramas with picturesque scenery, breathtaking shots and unique vibes

Chinese dramas, where FL actress is substantially older than ML actor (or female characters are much older than male characters)

Interesting dramas with smart FL

The leads know each other since childhood (not necessarily grew up together)

Vicious / vindictive / cruel / bad-boy type ML in Chinese dramas

Vicious / vindictive / cruel / bad-boy type ML in Korean dramas

Multiple ML's (3+) like FL 

ML / FL / SML / SFL have some disease. They might be poisoned or some obstacles restrain them from using their physical powers in full.