Didn't L get beaten by Kira eventually? I'm gonna say that Joker will beat L, as the Joker's mind is one of chaos - completely unpredictable and yet hyper-intelligent. I mean this is a villain who matches wits with the World's Greatest Detective on a regular basis. Also Joker is a far more capable fighter than people give him credit for - I believe he took out Luthor in Salvation Run.
L's a pretty cool character though, and Death Note's one of the few examples of manga I enjoy.