There are a lot of different opinions on what the best species and traits in Stellaris is, but here is my take on it. This guide is mostly for beginners and explains everything in detail. If you just want to read my conclusion you can scroll down to the bottom of the page. Anyways, first lets start with the species.
Selecting the best species.
There are a lot of pre-defined species you can chose from in Stellaris. At the moment there are 6 different groups of species groups that each race can belong to, that is the arthropoid, the avian, the fungoid, the mammalian, the molluscoid and the reptilian.
If you start the game for the first time in might be easier to select an already pre-define species, but I really prefer to create my own customisable species. Then I am able to select the government traits and ethos I think are the best. I suggest you do the same so you can do that.
Selecting the best ethos.
Your ethos is the sum of all the beliefs and ideals your race has. It is also referred to as ethics in the game. You choice of ethos will have big effects on the options your race has throughout the game, so it is important that you chose something you will enjoy playing throughout the game.
Your civilisation is also limited when it comes to how many ethics it can have. You can either have 3 moderate ethos types, or one fanatical ethos and one moderate one.
The best option I have found is to go with one fanatical ethos and one moderate one. The fanatical ethos I suggest that you go with is fanatic militarist. The fanatic militarist ethos give you a 20% bonus to army damage and 10% bonus to war happiness. These bonuses are huge and they will help you a lot when it comes to dominating your galaxy.
The moderate ethos I think you should chose is materialist. Materialist helps you get even more science and helps you grow your civilisation faster.
Selecting the best traits.
The last thing you have to chose are your traits. Traits can be selected when you start a new game, but you can also adapt new traits through anomalies or genetic engineering. These also affect the game quite a bit, but not as much as the ethos in my opinion.
Traits will affect things like production of population units, happiness, lifespan and other things. The first trait I think you should chose is intelligent. The intelligent trait increase engineering output by 10%, physics output by 10% and society output by 10%. These bonuses will help you a lot for sure.
The second trait to chose would be conformist. This trait help you keep your ethos by giving you a -20% bonus to ethics divergence.
The two negative traits you should chose would be repugnant and solitary. The reason you want to chose these two negative traits is that happiness does not really matter much. The only time happiness start to really matter is when it is above 80% or below 40%. Getting it above 80% is quite difficult, and keeping it above 40% is quite easy to do.
Choosing the best government type.
Your empire also have to select a government type obviously, and these give you other type of benefits or potential drawbacks depending on what you chose. There are 3 different types of governments you can chose from, the autocracies, the oligarchies and the democracies.
In an autocracy the power is held by a single individual, so this would be different types of dictatorships. In an oligarchy the empire is runs by a group of powerful individuals, a military junta would be a good example of this. In a democracy the power is distributed among the population and it is the way most countries are run today.
In Stellaris however, a democracy is not a good idea. The best type of government is in my opinion the science directorate. The reason for this is the important bonuses to research alternatives and empire leader capacity.
So to sum up everything, chose any species you want. Go with fanatic militarist and materialist for ethos. For traits you chose conformist, intelligent, repugnant and solitary. The government you should run is the science directorate.
I would love to hear what you guys think about this setup. If you have ideas for better combinations please let me know by leaving a comment.