Cobblers. They only came back to draw the game.
If we're talking underdog comebacks, hows this from the FA Cup 2015.
Chelsea 2
(Cahill 21, Ramires 38)
Bradford 4
(Stead 41, Morais 75, Halliday 82, Yeates 90+4).
Chelsea were reigning Champions and top of the Premier League again at the time...