Remembering all the hard nuts since I've been following Saints, the hardest has to be John McGrath. You knew saw him injured, or complaining to the referee. He got it dished out to him by opposing CFs and gave it back in spades. I remember another hard nut, Jimmy Case speaking in awe of John McGrath, as he'd been told stories of JM from fellow pros, who said being tackled by John was like being hit by a bar of iron.
John McGrath never sought to injure a player, [unless another Saint was being picked on] but he often did because of the severity of his tackle. He'd take the ball and man, there's no doubt. As mentioned earlier, he would come out of the player's entrance, dragging his toes as he trotted, with his shorts hitched up higher than the look of the 1980s, stinking of Vicks Vapour Rub [can't remember what it actually was], and he was always the last by at least a good 30 seconds. To my knowledge, he never came out with the team, To this day, I don't know why he did that.