A lot of people get ticked off with this discussion because they believe that there is a definite answer to this. There is no definite answer. It's all about who has the best argument. The writer Ohba said he thought L was the smartest, the artist Obata said he thought Near was the smartest, and 13 Death Note: How To Read puts Light as the smartest over L. I have even heard reasonable arguments that say Mello is the smartest. I personally believe Light is the smartest character. Even though L was fully convinced that Light was Kira, he managed to completely prove his innocence (even though he wasn't innocent) and then kill L with perfect timing. He would have taken out Near as well if it wasn't for Mello. Anyways, just give your honest opinion with a good argument:)
Update: You're a pretty good detective yourself, Yusuke. Spirit Detective that is.
Update 2: Actually, Misa Amane is a lot smarter than people give her credit for. She had several clever moments in the show like the way she secretly discovered who Kira was with that journal and the way she was able to confirm that Higuchi was Kira. The only reason she's called stupid by many of the characters is because she acts based on her emotions rather than what she should be focusing on. She's quite an intelligent character.