I don't really mind the types of personality the main female or male have, as long as they are well developed and well played.
However, I admit that I'm a sucker for quirky and cute characters. For both female lead and male lead. I like the ones who, when they fall in love, they are like the most romantic partners ever. Also, I like when they have quirks and unusual ways of thinking.
Example for male lead like that (in my opinion): Louis from Shopping King Louis, Enrique from Flower Boy Next Door, Byung Hee from Shut Up Flower Boy Band (okay, he's not exactly the male lead, but still is a cutie), Min Hyuk from Strong Woman Do Bong Soon
Example for female lead like that: Mi Ho from My Girlfriend is a Gumiho, Cheong from The Legend of The Blue Sea, Gong Shil from Master's Sun, Bok Shil from Shopping King Louis, Song Yi from You Who Came From the Stars, Dok Mi from Flower Boy Next Door