Hong Gil Dong. Not only is it my all time favourite historical drama, it's also my all time favourite drama period. Humour, heartbreak, action, romance, tragedy, that epic feeling of fighting the good fight for what you believe in, that knowledge that even if you die your legend will live on... It's such a brilliant beautiful drama. It's slightly cheesy, but coming from Japanese and Chinese dramas it doesn't bother me in the slightest. I love it so, so much.
I tend to be a huge fan of historical fans, so some of my other favourites include Iljimae and Bu Bu Jing Xin, with the latter having a slight step up from the former. Those are the three that just make me feel THINGS. Such strong things. Lol.
(Also, a comment above mentioned Splash Splash Love, which I wouldn't exactly count because it's time travel, but watching it was two of the best hours of television I've ever watched, no exaggeration. Just a joy and a pleasure and a privilege. Loved it.)