Is Lu Bu the strongest fighter on Dynasty Warriors 6?

Lu Bu is always the strongest in dynasty, he is the strongest in the whole story, there is noone stronger, even in books, no one can beat him.

Cao Cao beat Lu Bu. But in the game, he's not so good.

Depends on what you mean by stronger, if you mean in strength and just fighting, Lu Bu is unparalleled, but if you mean battle strategies and war skills, then yes, Cao Cao eventually beat Lu Bu with the help of Liu Bei. Lu Bu's generals betrayed him and fled for Cao Cao.