Summary
If you’ve ever seen an episode of Star Trek, you already have a pretty good idea of what certain types of federations in Paradox Interactive’s science-fiction strategy gameStellarismight look like. There’s a variety of organizational structures to choose from in the game that depart from this stereotype, from those focused on warfare, to religion, to research.
Whatever your taste, there’s a form of federation to fit your playstyle and plans for galactic domination. When two or more civilizations come together to form a single union, a federation is born. Being in a federation comes with certain bonuses, but also certain requirements.

6Galactic Union
The Baseline Federation
The humble Galactic Union is, in a sense, the more basic and therefore bare-bones version of a federation in Stellaris. A group or groups of empires forming together for their mutual benefit. At earlier levels, this will benefit any members of said federation primarily in additional cohesion and other diplomatic perks.
Over time, as the federation rises in level, members will reap the rewards of additional unity and enhanced damage against end game crises, the very threats that more often than not require groups of civilizations to work together in the first place. This form of federation is less specialized than others, and in being so, is less versatile for particular styles of governance but more approachable forthose new to the game.

5Holy Covenant
Uniting Spiritualist Empires
The Holy Covenant is the federation type geared towards spiritualist civilizations. This type of federation thrives on cohesion and spreading their dogma among other like minded civilizations. Members of Holy Covenants will have greater access to priests, which can then feed back into gaining further unity, and a boost tocreating pristine worlds.
As a religious institution, there are many requirements and regulations that disbar certain types of civilizations from even joining, meaning that this is a more niche option for very specific playthroughs. For those who seek to spread their ideology across the stars, however, this is an attractive option that curtails nicely with spiritualist empires.

4Martial Alliance
For Those With A Focus On Military Might
Sometimes, the best defense is a good offense. This is the principle on which the Martial Alliance is based. If this Galactic Union is the baseline type of federation, then the Martial Alliance is a version of that focusing more singularly on generating vast navies of more potency than they would otherwise create alone.
Most playthroughs of Stellaris, no matter the type of civilization, will almost inevitably includewarfare at some point, making this a tempting choice for those more inclined to indulge in the military arts. Whether you’re looking to defend yourself or take over the galaxy by force, this is a solid, if simple, option.

3Hegemony
For Those Who Rule With An Iron Fist
Second only to Holy Covenants, the Hegemony type of federation is perhaps the most niche. These types of federations are focused more singularly around a powerful central empire that holds the reins for all the others in its orbit, essentially rendering them as satellite states.
Though this requires more steps than simply taking them over outright, it is not without its benefits or flavor. This is another type of federation ideal for roleplay, and the benefits aren’t half bad either. Establishing the hegemony can be used as a war goal, and members of this type of federation will see increased levels of resources generated by jobs.

2Research Cooperative
For Those Invested In Technology
No matter what type of civilization you have in mind, all of them rely on technology to some degree. Therein lies the strength of the Research Cooperative federation. A civilization would be nowhere without their technology, and working your way through the tech tree is a core component of Stellaris' gameplay loop.
One of the main benefits of being part of a Research Cooperative are the boons to research agreements, allowing members to discover new technologies faster than they would alone. It confers benefits from having a high level of technology, and more easily allows you to discover technologies already owned by other members. As technology is the backbone of any good empire, this is very useful.

1Trade League
For Those Looking To Get Rich
If you want to control the galaxy, you could do much worse than controlling the flow of trade within it. Trade is a central part of the galactic economy, and leveraging this can leap to unprecedented boons. Capital can lead to an increase in all arenas, from military to technology.
In this sense, money will enable you to buy your way to victory, making Trade Leagues a very potent tool in the right hands. Trade value increases across all levels of this federation type, along with increased ship speeds. This might not seem like much compared to some of the other types, but should not be undervalued.