I once ran through all of Cosmic Encounter
's expansions and manually wrote down how many aliens from each expansion I wanted and I think that shows the difference in quality of the expansions. Hence why I own some expansions and don't own others:
Both Incursion and Conflict have some of the best alien designs in the game and compliments the base game very well. I would pick them in that order because Incursion is slighty better alien-wise. The actual "expansions" aren't necessary but I would add Rewards to every non-beginner game while I wouldn't play with all other expansions.
Dominion is great and has mostly fan-designed aliens. They are all very interesting but it tends to be more complex in either play or design. Great expansion, but I don't think anyone who doesn't have the former two should get it.
I own those three expansions.
Alliances is weak. Most of the aliens are weak designs or mere variants of other, more interesting powers. It's one of those expansions I would only add a couple aliens from because every mediocre alien that gets plays is one less Loser, Virus or Filch we could be having fun with. I would pretty much get it for the (original!) Schizoid, Butler and perhaps a couple here and there.
Eons was better received but I don't like. It has the original designers on board and it strove to push the envelope. For many that's fgun, but for me it resulted in aliens that were too complex and perhaps unnecessary. While some liked them, having powers with a "hand of small power cards" is not a direction I think Cosmic should take. I might get this expansion in the future but I' not attracted.
The most interesting part of this expansion are the "alliance dials".
Storm is awful. Most aliens in the set are downright bad and many have development issues. A large amount are just worse, wordier less strategic variants of other aliens. There are no aliens I would want to add to my game and some that are complete anathema to both game design and fun. Hey, have a power that can choose to switch places with you! All you did this game is useless and now I benefit! There are powers in this expansion that more or less do nothing. Awful.
Talking about which aliens are good: I now store my aliens in two of the expansion boxes inside the actual box. It's an obvious yet better way to store them than the handmade organizer I had and I can fit all aliens from the expansions I mentioned plus the base game. That's about 120 aliens! More than enough, I think. I also sleeve my aliens so chances are you can fit far more than I do if you don't.
While so far I've only cut one alien (Locust), I think there's value to only keeping the best of the best. Funnily enough, I thought about cutting Observers because it was "bad" only for it to keep winning!
By the way, I've been thinking about writing a bit more about the game. I think the world would welcome an article telling people why you shouldn't just invite everyone.