I can't think of a talented lead actor as most of them do fairly well but no one person stands above the rest.
As for lead actress, my favourite by far is Shin Hye Sun. She is absolutely incredible. Having watched so many Western and Eastern shows, I have never been as impressed with an actress as I am with Hye Sun.
Every show she has acted in has left a startling impression on me. Based on her already prodigious talent, she could have settled for a single genre and made tons of shows and gotten more famous and rich. But on evidence, she has taken the time to carefully select the right projects. And all of her projects feature characters that she has never played before. It is clear that she wants to challenge herself and I admire her all the more for it. As a testament to her dedication, she had even trained many months until she could pull off being a ballerina in Angel's Last Mission: Love. And judging from her tearful acceptance speech in a 2018 award which she received for her role in My Golden Life, getting to where she is now was not an easy route and she undoubtedly faced many disappointments and rejections on her way to making it as an actress.
The only thing that puzzles me is how I could have missed seeing this wonderful actress so late. I only got to know her through her role in Mr Queen which she knocked it out of the park. I look forward to her future roles with eagerness and anticipation of being amazed once more.