Strangely, this is the one I always think of too. An absolute thunderb*stard of a hit, with that satisfying/sickening from our perspective hit on the underside of the bar for good measure.
For Saints, Lallana v Hull or Lallana v West Brom Jan 04. That last one an under-rated one I feel, not for rat-boy's finish but for the way Ramirez laid the ball off to him leaving the keeper entirely out of the picture. The Villa game is oft discussed with Gaston but that was another sign of what might have been.