I think I haven't seen anyone mention 'Romantic Doctor, Teacher Kim.' Although romance is not the main point of this drama, their chemistry is off the charts, and we get to see that already in the first episode. Their kisses are definitely among the top 3 in my list.
Then, I second: Queen In Hyun's man, Another Miss Oh, It's okay, that's love, Marriage not Dating
And I feel recently they started to step up the game with kissing scenes in romantic dramas, like Because it's my first life, Suspicious Partner, I am not a Robot, Live up to Your name etc.