Probably Ruud Gullit for me. He was built like a proper number 9, but played on the wings/in attacking midfield, was a fantastic dribbler and possessed incredible workrate. His stamina levels were insane, if you consider how much additional weight he had to carry on him, compared to your usual winger/midfielder. Look at how Lukaku runs — and they are very much comparable in terms of their build.
Not to mention that he moved deeper and deeper as his career progressed, and played in central and defensive midfield before moving to the sweeper position. There are stories of ridiculous all-rounders, but most of them played in a pre-TV era — Sarosi, Valentino Mazzola, Zizinho... the only one we've seen (and not that much anyway) was Di Stefano, who also has a claim in this thread. But Gullit did all that in the late 80's/early 90's, when football was so different!