Re: who'd you go back in time to train with?
Japanese Arts? Myamoto Musashi (if he really exsisted the way the literature paints him)
American Martial Arts? Robert Rodgers (of "Rodgers Rangers fame...I'm a Ranger, what can I say :")
After that it would be a toss up between Jigaro Kano and O'Sensei as they refined things into Budo. I think it would be necessary to study with brilliant, raw tacticians before studying with these two, to appreciate what they would teach.