Garbage Fire Tier
I haven't really looked at gestalts or megacorps in too much detail but I've included the ones that stood out to me.
Slaver guilds is essentially 10% extra worker production from start. Not sure why people consider that garbage...
The lower political power for slaves also negates the decreased happiness. So that both non-slaver builds and slaver builds end up at around the same level of stability; including the resource bonus (~4% on homeworld).
Merchant Guilds: Probably the worst of the ruler civics in terms of what it gives you out the gate, but it gets good later on once you've unlocked better commerce buildings. It should really give a empire-wide buff like the other ruler civics. It's also one of those civics which is flavourful and good for RP too, which in my mind is worth extra points.
Inward Perfection: Yup, it's still really good, in fact it's probably even better now because growth is so important. The only problem with it is that you're basically stuck in German board game mode where you're just hiding behind your big starbases optimizing your economy and not really interacting with the galaxy. You can't even switch it out now, so no waking up when the crisis arrives and putting that economy to use. Nope, just building for the sake of building.
I find that Merchant guilds is great for both conquest and resettlement builds; You get slightly less amenities (3) and unity (1) than admins, but in turn it let's you produce energy and consumer goods (through trade policy) from the capital.
On the other hand, I think Byzantine Bureaucracy and even Aristocratic Elite have it beat in the end.
BB reduces amenities usage, and let's you build a building that - at base - gives about 16 extra amenities. I find that usually takes care of the amenities problem on conquered (AI) worlds.
AE on the other hand let's you add flat stability, which counteracts the happiness penalty you get from newly conquered worlds.
Noble Estates are also unlimited, and a great way to keep slave worlds over 50% stability.
Also you can switch out of IP by picking psionic ascension, then have your chosen one appointed as god-emperor.
As long as you don't have imperial authority already...