Great thread. Heroes, or role-models of any kind, are very threatening to authoritarian leaders because it challenges their narcissistic need for adoration.
A lot of very worthy people have been mentioned so far. Rather than merely adding names I'd like to riff on Mille's comment by qualifying as heroic anyone that takes a stand against unrighteousness whether they do so in a loud and vocal manner or if they do so with quiet dignity.
In fact, my preference is definitely for the latter variety. Too often individuals do something noteworthy but then start banging their own drum in attention getting behavior that to my mind detracts from the great good they do. In a very counter-productive way, they often, and (to paraphrase Ray Franz) with "depressing regularity," end up just like the leaders they condemn."