Without a doubt Tom Briscoe's on Fox takes some beating - the determination and technique he used was absolutely faultless.
As another Robin has mentioned Lynton Stott pulled off a beauty on a Salford player. Just to add to it, it was the 2003 season, IIRC we were on course to finish 4th whilst Salford had already clinched the Minor Premiership (and would go on to win the GF). It was the dying seconds of the game with us winning by either 1 or 2 points. Salford break away down the left and it looked a certain try, Stott desperately lunged at the player, angle-tapped up, bringing the guy to the ground and the impact knocking the ball free, with what must have been inches from the line. Absolutely fantastic, and moments later the hooter went to give us a great win.
There's one on that popular video site in the NRL where Gordon Tallis literally throws a guy around like a rag doll in the process of forcing him into touch.