I watched City Hunter and I loved how he went from a having fun type of guy to someone seriously trying to get revenge. I loved how he became calculating. Just want a drama with a mc who is carefree at first but then becomes calculative, and strong.

King2Hearts
Empress Ki
Faith (but you only see his "carefree" side in a flashback scene)
Innocent Man
 
I second King2Hearts. It's pretty good.
Bridal Mask is pretty much exactly that.