I can't find any video of it, but Garrison Hearst's ankle injury in 1999 versus the Falcons in the playoffs was brutal from what I remember.
OT: Worst Sports Injury Video
The worst one I've ever seen happen was Napoleon McCallum's knee injury in the 1994 MNF opener. His cleat got stuck in the ground as Ken Norton fell on his leg (sort of similar to the Theismann injury) and his knee turned inside out - his shin was almost up against his thigh.
Gross replay starts about a minute in.
This one is not only bad because of the inury, but also because the dude inflicting it was clearly stomping on the victim's leg and then questioned the red card.
Wow, soccer players flop way too much! /s
srsly that was disgusting.
shaun livingston's injury will always be #1 to me.