Warcraft lore seems to fall into two distinct categories: the things you see in the games, and the things that have been established through story alone. While I certainly won't profess that I hold a PhD in Warcraftology, I've logged enough hours in World of Warcraft off-and-on over the past nine years to make my parents wonder where they went wrong.
While I'm not the guy who will be able to provide a dissertation justifying all of my choices--nor will I be able to comprehensively compare and contrast the merits of Grom versus Rexxar--I learned more about the world of Warcraft than I ever cared to know. So here's your list, guys.
And spoilers: Thrall didn't make the list. Controversy!
Velen didn't seem to make too many of the top 5 lists that I used for research, but after reading his biography, I don't quite understand why he's usually excluded. He was a peer to two of the most powerful Dreadlords (and the only one who didn't fall for Sargeras's deception), and he is freaking ancient. His longevity has to count for something, doesn't it?
He settled his own world, raised a race of his own people, and oversees the Draenei capital on Azeroth. Oh yeah, and he has psychic powers! I'm glad he's on our side, basically.
She gets a boost just due to her character being Arthas's old squeeze, which makes her the perfect archetype for someone who ranks pretty highly (but not too highly) on a list like this. Her entire home and its citizens were destroyed and she's on a mission for revenge, so that also helps her on a list like this.
The pick who is so obvious that he can't be left off the list, but Varian Wrynn is also so obvious that he can't get placed too highly, either. I'm completely prepared for someone who knows way more about this than me to tell me I'm a complete idiot for saying this, but Varian Wrynn sure seems like the Isildur of this world. The hero with a huge role and devotion towards the race of man, the epitome of alpha, and a man with a great sword. And, according to multiple entries into the canon that I cannot even begin to understand, he sure is great.
So many of the possibilities for the greatest heroes in Warcraft appear to have been around forever, in addition to being immortal, so it's hard to give someone too much credit for that, but it always feels different when an elven character does that. There's just something about them that feels so sage-like, and Malfurion continues that stereotype. He's all-wise, and the foil to his hotheaded but ultra-powerful twin, Illidan. He also basically destroyed Archimonde in the Third War, so score another one for elf supremacy in the fantasy genre.
There seems to be some controversy over whether or not Broxigar should be placed in a list like this due to the fact that he is in the stories only, and has never appeared in the games. However, even as a guy who has zero interest in Warcraft lore and whose only experience with the series has come through World of Warcraft and playing Warcraft III when I was 12, I still can't deny Broxigar the top spot after reading his story. To sum up his entire canon: he defends the world tirelessly, then feels so guilty that he is the last of his army to not have died with honor that he goes to get killed by Sargeras himself. But, the dude is so coldblooded that he wounds Sargeras first, which apparently is a massive, massive deal. Beat that, Thrall. Go give Garrosh another promotion.