LogFAQs > #896338295

Oh, it's probably worth mentioning that Stellaris and its expansions are on-sale in multiple places, including the (essential) Utopia expansion at -50% off. In fact, the Paradox Store offers it as a 3-for-2 option(presumably the cheapest eligible thing you pick up is free).
https://www.paradoxplaza.com/stellaris/STST01G-MASTER.html
Of course, everything from EU4 to CK2 to HoI4 to Tyranny expacs are eligible.

As far as Stellaris expansions go...

Must-Have
Utopia- Utopia adds Ascension Perks. Although Ascension Perks will become a base-game feature as of the 2.0 patch, the Biological, Technological(Synth), and Psionic Ascension Paths will remain locked by Utopia. It also offers Orbital Habitats, hive minds, and megastructures.

Good
Leviathans- Leviathans reworks exofauna(space monsters) to appear in clusters and adds a number of terrifyingly powerful uniques with varying rewards for killing them. It also adds special outposts. These outposts are inhabited by aliens who have been pursuing business, science, or the arts since time immemorial, and offer a variety of perks, for a price, of course.

Niche(but good)
Synthetic Dawn- This relates entirely to Machine Empires. You can make machine empires, in four flavors. The default machine empire type is similar to a (regular) hive mind, but the three variants are Determined Exterminators(they seek to eradicate all organic life), Assimilators(they seek to forcibly cyborgize all organic life and integrate it into the machine intelligence), and Rogue Servitors(robots who seek to "protect" organic lifeforms by confining them to lavish preserves where they are aggressively pampered). It also adds an additional Fallen Empire type(a machine empire seemingly designed to protect organics.....but from what?)

Pretty(but no game-play ramifications)
Plantoids Pack- Plant people portraits, complete with their own city art and ship set. They're pretty good.

Humanoids Pack- Star Trek-style "rubber forehead" aliens(including dwarves, orcs, and a cyclops!), as well as unique city art and a ship set.

Free
Horizon Signal- An event pack written by Alexis Kennedy(Sunless Sea). Primarily centered around the eponymous event.