World of Warcraft: I hate it, but I know it changed online gaming and has made mucho money. Yeah, it gets it.
Diablo II: For the same reason as WoW, it's still played today and the vast array of possibilities with each class are amazing. And unlike most MMORPGs, the combat design allows for more tactical play that point and click.
I would kinda agree that Dawn of War should be there more than the second, BUT it really came in small bits which did nothing more than starcraft did. As a whole it was much more impressive.
Homeworld 2 I would be inclined to agree with because I'm a fan, but I get why they didn't do it. It didn't push the boundaries beyond what Homeworld did and it had fewer tactical possibilities than the first, and most certainly fewer than Homeworld: Cataclysm. Not to mention that, unlike Sins, it is not designed to be modded, thus projects like Homeworld: Complex have to stand on their own.