I liked You who came from the stars, the first time I watched it. The second time was a little meh. Jun Ji Hyun carried that series with her amazing acting. If you'd put in another female lead and that series would have tanked probably.
But my worst series I've watched is Backstreet Rookie! It's disappointing in two ways, both the leads performance disappointed me because they are among my favorite actors, but the story is just plain boring... it's almost about nothing. If I had to describe the series it would be "takes place in a convenience store". That's not very interesting.