A little late to this discussion but here is my opinion. I care more about DLC that expands the sandbox mode, not about campaigns or cosmetic things. Though if you guys port over the campaigns of the previous GC games I won't complain.
DLC ranked by how much I enjoyed them
1. Mercenaries - it's great to see the Arceans and Torians again. The mercenaries are also interesting, although I feel there could be more of them. The campaign, really it's just one scenario, feels rather short, however I do like the events in it. I kinda feel 20$ is rather high for what if offers, maybe 15$ would have been better. Also the AI doesn't seem to purchase mercenaries that much. In my current game only one AI has hired a mercenary.
2. Revenge of the Snathi - I really like the Snathi because they bring something new to the game, their Scavenge ability. The scenario while rather short it was nice to have a different perspective other than the Terrans. I would like it if the Arnorian Wardens, I really like the design of their ships, were somehow added to sandbox mode.
3. Mega Events - I enjoy things that can shake up the status quo. Although I think this DLC had the misfortune of causing confusion because of its the name and people not realizing that several mega events were added to the base game in a patch.
4. Lost Treasures - new colonization events with interesting rewards. What's not to like?
5. Rise of the Terrans - this got to #5 because I'm not interested in making maps or designing ships. The "new" civilization is just a reskinned Terran Alliance, they even have the exact ship syle. The campaign was rather good, although I would have preferred if the Xendar and Stellar Guild would have at least gotten a face. I would also like it if the Stellar Guild and Xendar were added to the sandbox as minor civilizations.
6-7. Builders Kit & Map Pack - I'm just not interested in creating maps, since I prefer random games, or designing ships, I care more about what components they have than what they look like. Although I acknowledge that there are people who do enjoy these things.
Soundtrack - this isn't ranked since doesn't actually add anything to the game but I enjoy listening to the game's soundtrack.
One last thing I would like to state. It is my impression that Stardock believes what players want from DLC are new campaigns, this is based on several statements from Brad and what the next scheduled DLC are. At least in my case this is not true and I believe that the Snathi DLC is the best selling one because it adds a new major civilization with a new and interesting ability, new tech tree and a new ship style.