Hands down, Kakashi is the best character. He's laid back to the point of seeming lazy, and he's always late, as if he lacks motivation. He comes across as completely irreverent and indifferent, demonstrating no compulsion to fight honorably. For example, he reads a cheap novel during training combat and even blows off a challenge from his rival, Gai, by turning their encounter into a game of rock, paper, scissors. He likes to toy with people by playing dumb or countering others' enthusiasm with his trademark indifference (as he often does to Gai). A good demonstration of Kakashi's twisted sense of humor comes from a very bad filler episode in which Naruto, Sakura, and Sasuke try over and over to make him take off his mask. He remains one step ahead of them the whole time, but he pretends not to catch on, and at the very end of the episode, they ask him straight up to take his mask off. In response, he slowly removes his mask--building up their suspense--only to reveal another mask underneath.
Kakashi is all about subtlety and understatement, and he's not easy to figure out. When he first appears, he just seems incompetent. In time, however, you find out that he is one of Konoha's greatest ninjas. But you have to look past his chronic dumb stare and his two layers of masks to understand just how cool and complex he really is, and that makes him the most interesting and engaging character of Naruto.