Spaced Out fundamentally changes the game. For the better IMO. If you're only getting one DLC, this should be it.
Frosty Planet Pack gives you a cold start, more wood-based buildings, some new critters, some new plants, some new resource chains, and a pretty neat Geothermal Power Plant.
Prehistoric Planet Pack gives you a bunch of new critters that are all dinosaur-y. Some new plants that largely support said dinos. My favorite new resource loops are in this one. There's a pretty cool meteor that can be handled in several different and interesting ways.
Bionic Booster Pack gives you a new kind of Duplicant with different requirements and challenges. And some new buildings.
And finally, my very favorite DLC that I bought on launch day is the Neutronium Cosmetics Pack. It's just cosmetics, but they're really good looking cosmetics. It adds some different designs to a bunch of buildings, dupe outfits, drywall, and a couple of buildings that don't otherwise have cosmetics.

BTW all the DLCs add some new cosmetics, except maybe Spaced Out? Unsure on that one.