1. Green Planet expansion. This is certainly the best expansion that greatly enchances and prolongs the game by providing a long-term goal. The terraforming of Mars is a complex, but rewarding process. Otherwise, everything becomes rather boring and repetitive after you've set up all the production chains.

It's the only one that's worth the full price.

2. Space Race expansion. It adds rival colonies, unique buildings, vehicles, new factions, and a trade function. You will also have to compete for the milestones with the other colonies now.

This certainly improves replayability, but is not a necessity. Get it on sale, or as part of a bundle.

3. Colony Design Set and In-Dome Buildings Packs. Those DLCs add more buildings and a few skins. The balance varies, the retirement home and school spire are too OP, while the hospital and large oxygen tank are very useful (and should've been in the base game).

With some buildings being useful, but others way too OP, it's not a neccessity as well. Again, sale or bundle.

4. Project Laika DLC, quite useless. The ranches are an unimaginative, unrealistic, OP and mostly cosmetic "alternative" to farms. The pets do nothing but wander about and are just plain annoying.

Not worth it to buy individually, but if it's part of a bundle, then why not. However, since those ranches don't need specialists or animal feed, it's not only OP, but also somewhat immersion-breaking.

5. Below and Beyond "expansion". Just stay away from that pile of crap, its buggy as all hells combined and requires ridiculous micromanagement efforts.

Just don't buy it. Even if it's part of a bundle, deactivate it. Neither the "official" nor the community patches can guarantee that it works properly. Since Paradox has abandoned the game, and this DLC doesn't seem to be too popular with modders, the chances for official or unofficial patches are nil.

6. The various cosmetic DLCs are entirely optional, and add purely superficial stuff like radio stations or dome/building skins.

Just wait for a bundle or simply ignore them, unless you really need that little bit of eye (or ear) candy.

7. The newest "Mars Express" DLC. Well, it's a nice concept, but since the train tracks are limited to flat terrain only and can't scale elevations, it is a far too limited transport option. Furthermore, there are no hub stations and everything is rather micromanagement-heavy. At least the bugs are fixed now.