Lee Min Ki and Seo Hyun Jin - The Beauty Inside (one of my all time fav chemistry)
Yeo Jin Goo and Park Gyu Young - Dali and Cocky Prince
Lee Joon Gi and Moon Chae Won - Flower of Evil
Lee Dong Wook and Wi Ha Joon - Bad and Crazy (that one scene had me sobbing fr.. if you know, you know)
Lee Joon Gi and IU - Moon Lovers (timeless drama)
Kim Tae Ri and Byung Hun - Mr. Sunshine
Kim You Jung and Song Kang - My Demon
Lee Min Ki and Nana - Oh My Master (again, another all time fav... you wouldn't regret it)
Seo In Guk and Jung So Min - The Smile Has Left Your Eyes (and another one)
Ji Chang Wook and Shin Hye Sun - Welcome to Samdalri (but honestly, any drama with Chang Wook is bound to have top tier chemistry... one of the first actors I was introduced to and the only actor where I've seen all his dramas)
and of course the ones that were already mentioned... CLOY, Goblin, The Crowned Clown, Her Private Life, Love To Hate You (especially love this one, just different than any other Kdrama I've seen), Vincenzo, It's ok to not be ok