I think this thread is turning into the thread I'd been planning to make recently, asking people to talk about some of their favorite "overlooked" campaigns that don't get included in the Derelict/BP/Transcend/etc. "What's the best?" listing. I know I've probably overlooked a few gems myself, and it'd be good to spread awareness about some of the works out there that don't get mentioned that much.
Among many others, I'll throw in a plug for the Ancient-Shivan War's initial release myself. While it is true that the missions don't do anything remarkably out-of-the-box, and that the Act 1 campaign is relatively short, it's still a solid and fun experience. What really stands out, however, is the overall atmosphere, which is flat-out incredible. You genuinely feel like you're a member of an ancient, powerful, arrogant species who views the entire galaxy as being their sole dominion. The prose, the interface, the music, the overall tone of the writing...they're all top-notch. The old Ancient fleet models are used to great effect, and given a shiny new coat of paint to boot. It was one of the more immersive experiences I've had in this game, and I can't wait to see how the team follows it up.