Faith because it was the first Drama I watched. Then BOF, Heirs, City Hunter and last, but, i still thoroughly enjoyed it...Personal Taste. Sorry, I'm a LMH fan so i just had to mention more them all.
I believe that the most complex role was the one from City Hunter, so that was definitely my favorite LMH drama. The role of general Choi Young didn't quite suit him, for his first historical role he could have chosen something similar to Yoochun's character in Sungkyunkwan Scandal; as for Heirs... I hope the drama improves (writer-nim, I'm looking at you!)