This is a really hard question. All of them died (Leloch is not comfirmed, but i doubt that he would survive and then just go underground never to be seen again). Light was responsible for L's death but it was because of the death note. L don't have any powers that could have helped him so i don't think it's fair to judge from who died first. Lelouch showed that he was smart many times but also hes plans often failed. Light was the master of manipulation but got outsmarted by Near and Mello who was L's plan B. L did always suspect that Light was Kira even if he got evidence for the opposite. L and Light showed that they were smart much more then Lelouch also.
My final verdict is that L is smartest.
*Note though that this is only my personal opinion and how i see it, you do not need to yell that i'm wrong if you think differently from me.