I can’t really choose between all the dramas, but here are some of the best ones I’ve seen up till now.
Fantasy based: My love from another star, My girlfriend is a Gumiho, Orange Marmalade, High school Love on.
School: The heirs, School 2015, Reply 1997, Dream High, Sassy Gogo.
Old dramas: The Moon that embraces the sun, Heartstrings.
Gender-bender: You’re beautiful.
I find that k-dramas usually have the same plot, and the most common ones are “the rich guy falls for a poor girl”, “Living together”, “The bold and justiceful girl”. If you are somewhat for d of these, you should go for fantasy based dramas.