Two, Five or Eleven.
Maybe Nine as well.
I suspect I'd want to punch Ten if I travelled with him for any length of time. Similarly with Six. Seven I'd be too afraid that I was only being taken along as a pawn in some grand Machiavellian scheme. Eight, meh, Three would be too superior all the time and One...as has been said, I think the First Doctor, particularly a pre-Ian/Barbara Doctor, might be the most dangerous one of all.
Four's the only one I'm not sure of, on the one hand he might be very cool, but on the other travelling with him might prove as annoying as travelling with Ten or Six.