A lot of good ones are mentioned already. Although I'll add some descriptions in order to help you decide.
"Queen In-Hyun's Man" is heavy on the romance in ways that other dramas don't come close. The leads are sweet and adorable when their together, and till the last episode you are not sure whether they'll end up together or not. There are great fight scenes, an interesting historical world, and one of the few time-travel dramas that deal with the consequences of messing with time.
"Faith" is one of those dramas that you'll either really like or just won't. When I first watched this drama, I found the female lead to be annoying. As the series went on, she grew on me. She was the comic relief in many serious, formal scenes and there is a great chemistry between her and Lee Min Ho. The Queen and King of this drama has an interesting relationship and they are so well-acted that lots of people end up watching the drama just for them. It's far from perfect though; the fighting scenes aren't as well-directed as they could be, and that leaves the fighting scenes to feel not as life-and-death (and therefore not as exciting) as they could be. It's also heavy on the scheming and political maneuvers against a comical-looking (if a bit one-dimensional) villain. It is, however, one of those few dramas that made me laugh in every episode, so I whole-heatedly recommend this one.
Startling by Each Step is, without doubt, one of my favorite dramas. The cinematography is beautiful. It feels like a sageuk, with picturesque settings and beautiful costumes. The female lead adapts almost too easily to being thrown back in the past, which I found kinda refreshing from the usual stumbling and dense female protagonist. The first half of this drama is very light-hearted and funny, but it progresses to more serious as she transitions to working in the palace (where she has to watch her step closely). Heavy on the politics, with a mature female lead, and interesting interactions with all the princes (there's more than 13), this is definitely a great drama to watch.
