The base game itself is quite lackluster, unfortunately. It is nice, of course, but has low replayability value. You may squeeze out a few more playthroughs by raising the difficulty level, but sooner, rather than later, you'll get sick and tired of yet another dust storm.

You see, it is Paradox' policy to release a "teaser" of a base game, then milk the players with countless DLCs. Surviving Mars is not quite as bad in this regard (at least compared to CK2 or Stellaris), but it still needs a couple expansions for a proper experience.

Well, you don't need them all. Long story short, the Green Planet expansion should be the bare minimum which to buy, and it's the only DLC actually worth its full price. The rest are optional, not a necessity.

If you don't want to spend too much, then wait for a sale, and get both the Green Planet and Space Race expansions. This will double or even triple a single playthrough's length, and also provide more (much-needed) replayability. If you have a bit more money to spare, get the Colony Design Set and In-Dome Buildings Packs, too. The best decision, however, would be to wait for a package sale or a HumbleBundle that includes all or most DLCs. Just make sure to deactivate Below&Beyond, it's too bugged.

You can also look up any deals here: https://isthereanydeal.com/ and the links to the various packages are provided in the last section. I recommend getting the Season Pass pack.

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. Consider it a gimmick.

Just get it on sale, or as part of a bundle, unless you really like trains.

There are several Packages for this game, which contain a number of DLCs. Buying them may be more beneficial than picking the DLCs by piecemeal, especially if they're on sale. Of course, the GOG versions can't be used with steam, and vice versa.

Deluxe Edition/Deluxe Upgrade Pack: This one contains only more skins, a radio station and an art book. As the name already implies, its pure luxury.
https://isthereanydeal.com/game/survivingmarsdigitaldeluxeedition/info/

Starter Bundle: A rather weird set to start out with, this comes with the Deluxe Upgrade Pack, Green Planet, Mysteries Resupply pack (which is free anyway) and the In-Dome Buildings Pack. Unless you find a very good deal for it, better pick the Season Pass instead.
https://isthereanydeal.com/game/survivingmarsstarterbundle/info/

Season Pass: This is the best package, since it has all the major DLCs with the sole exception of the In-Dome Buildings Pack. Contains Space Race, Green Planet, Project Laika, the Colony Design Set, and finally a radio station (Marsvision Song Contest) as an extra. Interestingly enough, this one is no longer available on steam, but you can still buy the package somewhere else, and its key activates here just fine.
https://isthereanydeal.com/game/survivingmarsseasonpass/info/

First Colony Edition: This is simply the Season Pass plus the Deluxe Upgrade Pack, but still without the In-Dome Buildings Pack.
https://isthereanydeal.com/game/survivingmarsfirstcolonyedition/info/

Complete Colony Bundle:
Contains everything. Unfortunately, this also includes Below&Beyond, which I recommend against.
https://isthereanydeal.com/game/survivingmarscompletecolonybundle/info/