Shikamaru may seem very lazy. According to Shikamaru nearly everything is troublesome. If it was up to him he would spend the entire day lazying around and playing games like Go and Shougi, and he's really good at it too. His sensei Asuma believes that he will one day become a genius military leader.
Shikamaru is a really easy-going fellow. In school he got about the same grades as Naruto had, but this was simply because he was to lazy to even pick up his pencil. The truth is that Shikamaru is highly intelligent. His sensei secretly did an IQ test on him once, telling him it was just a game, and the result was that he had an IQ of over 200 (which is exceptionally high).
Knowing this it is safe to say that Shikamaru's most powerful weapon is his intelligence. He analysis every situation carefully, and always thinks ahead before doing anything rash. Combine this with a very cunning use of his best ability, the Shadow Copy, he is not to be underestimated.