Hitler.
Karl Jung.
Muhammed Ali.
I'd let Jung grill Hitler for an hour, see what makes him tick, then gave Ali go 12 rounds with him, bare knuckle.
Seriously - I'd genuinely like to see Hitler in the flesh, see if he really was as charming as he was made out to be, enough to make sane men do unspeakably evil things to please him right til the end when it was clear he was beaten and broken.
Jung, always fascinated me, his views on some of the drivel Freud came out with echoed mine when I first started Uni.
And Ali, I'd just like to have a matey chat with him, the peak Ali at his braggadocio best, before the Parkinsons got to him. It's not often an arrogant man can be so loveable with it, he had the ego of Trump, with none of the dislikability.