I think you should start with Pinocchio. It was my first drama and my eyes got caught by him. Then I would say While You Were Sleeping.
I Hear Your Voice is a good drama, but the main couple has an age gap, some people find it weird. The story is more than just this couple.
Then maybe, if you REALLY love him, try W. I think it was a very confusing drama, but he can deliver some pretty good kiss and romance scenes. But I think he is a good actor overwall, I dont see him as anything but as romantic male lead type.