I'll go with Farve too. Toughness is more than merely physical toughness. Its mental toughness as well, and after that Monday night game this past year I can't think of anyone tougher in the NFL.
On a side note: Where's the love for Donovan McNabb? The man played an entire game on a broken leg, and led his team to victory. Granted, it was against the Cardinals, but an NFL team nonetheless. That at least deserves an honorable mention.