Lee Dong Wook and Kim Sun Ah in "Scent of a Woman". This was a drama I didn't want to invest much time in for watching, so I was just reading recaps on dramabeans and skip-watching the episodes. Then I got to their tango scene with her in a red dress, and their chemistry struck my like a ton of bricks. From that moment, I started to watch properly, without skipping any scenes, and I even went back to watch the previous episodes fully. So this drama is, until this day, very memorable for me.
"Business Proposal" and "Queen In-hyun's Man" had fantastic romantic chemistry.
Also "Junior High Student Miss A" (drama special), "My Demon", "My Princess", "The Beauty Inside".
Oh, and "Alchemy of Souls" season 2, and maybe "King the Land".