He is my favorite DN character and favorite anime / manga character, overall, but I adore Matsuda-kun, as well. ^^

Overall Favorite Male - L Lawliet

I do rather love both of these characters but I have to say that I favor Mello slightly more, due to "Another Note".

Matsuda is my favorite and I dislike Mello, so this was an easy pick. xd
Even though I quite enjoyed Mello in 'Another Note'..

*Spoilers about the ending ahead*

I can't really give a valid opinion on Mello, since I've never read Another Note and everyone says he's more likable in that, but from what I've seen I find him annoying.
On the other hand, I really like Matsuda and thought it was brilliant when he shot Light. :D

Matsuda had more character build up. He takes charge in episode 37. After confronting Light in anger for what he'd done. It made me realize that he wasn't indeed stupid. Though I wish, Kiyomi would've fooled him. And had sent a guard of her's. And Light would've failed. Mello wanted to do that. But Light wouldn't kill his sister.

L, Mello and Matsuda are my 3 favorite