There's been plenty but I'll go through a few that were surprisingly good.
Let's Eat - I didn't really know any of the cast. I don't like food dramas. The trailers were... meh. But since it was about single people and neighbours, I was intrigued and had to check it out... And so, I loved it from the very first episode.
Once Upon A Time in Saengchori - Again, didn't know the actors and wasn't even too sure how that episode 1 made it's way to my screen. But it was so good!
White Christmas - I heard rumours that a certain drama special was gathering a bit of cult reputation. But a school for genious kids? Sounds dull. But after I gave it a try? Woah.
She is WOW! - Weird plot summary but decided to try it out. And suddenly I was hooked.