For me it's Diablo II. Granted I've played my fair share of D2 since launch, and also recently on a private server with a comrade from hexbear, but I still feel like years later the game didn't grab me as much as D1 did.
Granted I don't hate D2, but for a game that I keep coming back to, D1 takes the prize.
Personally, I think a campaign interface that's the Dark Crusade and Soulstorm campaigns combined with aspects of Total War (like unique buildings suited for that province, separate from the main RTS battle buildings) would be ideal. Could expand the scope to be multiple star systems to really get the "grand" scale.
And by guilds, do you mean Avatar Conquest from TW Shogun 2?
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