my #1 is Empress Ki for sure , I was paralyzed by the events for weeks even after I finished it.
1 Litre of Tears, Mask, Boku no ita jikan , 49 Days are all amazing yet so sad dramas (all managed to make me sobbing)
Six Flying Dragon, Orthro no Inu , Secret , Lipstick are also 2nd of the list (all are amazing and a somehow sad drama)