Plantoids is a species pack, adds new portraits, new ship models and name lists, but nothing gameplay-related. Nice for make sure larger galaxies are diverse, but nothing you need.
Leviathans is, IMO, the weakest of the non-major DLCs that add new content, but is still good. It adds the titular Leviathans, space creatures and ancient tech that create a sort of late mid-game challenge for fleets with some pretty nice rewards (like new techs and your own space dragon) Second bit are the Enclaves, independent stations that offer services to those who don't want to blow them up: Traders till will, untill 2.2, exchange food/minerals/EC and offer strategic resources, Curators (which also show up in Distant Stars) offer research bonuses, hints and buff vs. The Leviathans, and a scientist with unique traits that make some of the rarer rare techs more likely to show up, and artist enclave gives you a unity buff, while having several random events and planetary buildings/upgrades for you to buy for ethics and unity. Finally, there is the War In Heaven, a sort of End-game crisis (that doesn't actually count as one, Unbidden or the others will still show up) where a second FA will Awaken after the first does, and then starts a massive war, trying to force the regular empires to join one side or the other.
Multiplayer is Multiplayer. Word of advice: multiplayer in Stellaris is best if you are working co-operatively with other players or set hard ground rules as for what can go (like, only a single planet can be taken in a war and no wiping out each other until War in Heaven or the Crisis hits) competitive no-holds-barred multi for Stellaris is an exercise in frustration, just too many underhanded ways to wipe out a neighbor before you even get to destroyers and to self-sabotage to try to make said conquests worthless.
Finally, Utopia. First Major DLC and IMO, still the best. Biggest draw are the Acension Paths, ascension perks you can take to let you gene-mod your pops with impunity, turn yourself into robots, or access new Psionic mechanics with The Shroud. You also have the first set of Megastructures, multi-stage space installations such as a Dyson Sphere and your own Ringworlds. For the morally dubious, there are new slavery and purge types, such as forcing xeno pops to work to death or to keep them as livestock. Finally, last but not least, you have playable Hive Minds (which is now a special ethic and authority type, plus unique civics, including the home-grown Pretheryon in Devouring Swarm) and Fanatic Purifiers (which is it's own unique civic, giving massive bonuses to your ships for the cost of everyone hating you, for good reason) also civics for starting with robots (which is quite powerful in pre 2.2) and starting with a second servant species (which is not so powerful until 2.2)