I have over 1200 hours on EU IV, 800 hours on CK2, have owned and played EU3 since before it was even on steam (for that matter, I owned CK2 through Gamersgate and own EU3 on CD), have owned and played probably close to 3000 hours of Civ IV, some 500 hours of Civ 3, probably close to 1000 hours of Master of Orion 2.
Stellaris is a 4x game made by Paradox. You are a complete idiot if you think that Stellaris will not play out in a similar fashion to a 4x game made by Paradox *BECAUSE THAT IS EXACTLY WHAT IT IS*. There are certain lessons you learn when you have experience with 4x games, things like food being super important, avoiding long and costly wars at nearly any cost, and that science and production are the things that matter most but every game has different ways to give you these.
If you don't know enough about food and why it is important, watch literally any serious Civ III playthrough and get back to me. Or even Civ IV- Sullla did a great Dutch playthrough that would be very instructional to someone such as yourself, but you'll still need to take the extra step of applying the lessons you learn to new information, a skill you should have developed around the time you learned human speech.
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So, let's address the specific ethoses. Pacifist gives +10% food. That's incredible right there. An ethos point is a little more rare than a trait point, so is supposed to be marginally better, but this is way better. The 1-point trait production bonus- and there is only one of them- is Strong, which only grants a +5% mineral bonus. The fact that food is incredibly powerful, combined with the size of the boost, make up for any semi-visible weaknesses of Pacifist (the war weariness penalty is probably nothing, as war exhaustion probably acts a lot like war exhaustion in other 4x and Paradox games, so just avoid long wars), such as the bombing in support of thing. Also the Pacifism governments are ridiculous. Also do note that your feelings about Pacifism will largely correlate to your feelings on food, but if you feel as though food is not super powerful you aren't paying attention.
On the other hand, Militarist does not boost anything. It prevents war exhaustion, but so does winning. This doesn't help you win. It helps you not lose, which in competitive games is not the same as winning. It doesn't reduce ship cost or upkeep. It gives you nothing in exchange for that ethos point. The only thing that is good is the access to Military government forms, but even those abilities are a little meh. It will depend a lot on how the bonuses from having a military ruler instead of a civilian one work out.
Spiritualist gives a +5% bonus to happiness. Now, this is not a +5% production, where if you are "making" 50% happiness it only goes up to 52.5%. It is a literal +5% boost per ethos point, representing a lot of moves from say 60 to 65 (check the Blorg stream for how rough happiness penalties can be, though recall that Wiz is running repugnant with Xenophile, a combination guaranteed to be terrible). Judging by how many empires have been having instability problems in the stream, as well as the fact that happiness presumably will help you run slaves without needing to run Fanatic Collectivist, Spiritualist is very powerful. Additionally, it is not impossible that a high happiness would actually boost productivity, in which case a big happiness boost is not only a way to stabilize an empire and enslave it, it could act as a synthetic source of research points.
Materialism, on the other hand, doesn't really do anything other than give you robots and Science Directorate. Science Directorate seems pretty bonkers- just imagine how much more smoothly your tech progression would be if you could just pick whatever tech you wanted to instead of the best of what is available one month. Obviously moving from 3 to 4 cards isn't *infinity,* but it is a 33% boost. I imagine that this boost will feel big enough that it will actually enable lots of players who want to play science to really play science. It seems a little squishy, as it can only run Collectivist and not Fanatic Collectivist, and can't use Spiritualist to off-set happiness problems from slavery, so it either needs slavery and Collectivist and a heavy hand to control the population, or individualist for that nice +10% energy production. In any case, Materialism only gives +5% science, which judging by the fact that you can find it literally anywhere in the galaxy not being produced by your pops means that it is basically irrelevant.
Collectivist and Individualist are both complicated stories due to the fact that we haven't seen a lot of things related to slavery, but Collectivist gives you a nice -%food required bonus and assuming that this operates as a real food modifier and not just a growth rate modifier, Collectivist feels like the more powerful of the two. However, Fanatic Individualist did show us a +50% energy planetary edict, which should give you enough energy to settle your first couple of worlds right there (starting energy on homeworld seems to be about 8/month, so this is +4 per month for 10 years, which is 480, whereas each colony ship was costing 7.5 energy per month for 12 months, or 90 energy, so this edict represents 5 colonies worth of energy on turn 0). Energy is a lot like food in the sense that you want to waste as few tiles on these resources as possible, so boosting it seems neat, but you can just find it in the galaxy so I am slightly less impressed by just Individualist.