I'll try to add without repeating the ones that have been mentioned.
Beautiful Gong Shim- ML falls head over heels in love with her and pretty quickly too. It takes her a little longer to fall in love with him.
Boss and Me- He starts to have feelings for her without even realizing it, he loves to watch her eat.
Doctors- He falls for her early on. She's his student so he can't pursue her but, when they reunite after she grows up he actively pursues her.
Scarlet Heart: Ryeo- She falls in love with someone else while he falls in love with her. He's an antihero though, so not fluffy romance with him.
Ouran High School Host Club- He's crazy but so in love lol if you like the anime-to-drama and its ridiculous situations.
Pinocchio- He's in love with her since they were young.
Reply 1997- Both leads fall in love with her first, you have to wait to know which one she chooses.
Secret Garden- He pursues like right after meeting her.
The Eternal Love- Not Ten Miles of Peach Blossoms (lol)
I think The K2 falls in there somewhere but, he's an antihero not a good boy. He does fall in love first, though.
