I was personally a bit disappointed with Owls, even if the general consensus seems to be that they are strong.
The early extra yellow card is great, but more often than not I find that I can make do without it (without it affecting me too much), and sometimes I even don't find any proper strong cards to slot there (making me for instance slot extra builder production in, even though I'm not focusing on builders now, but other options are even less attractive).
And the value of extra card slots do fall off over time, as the relative increase in cards compared to new governments is less than what it is in the Ancient Era (also the fact you can get card slots through certain wonders if you really need them).
Vampires on the other hand, I'm very impressed with.
Getting an extra unit out is great for my playstyle (I often tend to take out my nearest neighbour early if I can), and the fact that it automatically scales in combat strength
and stacks every kill into additional combat strength, makes the unit properly OP in some early war scenarios.
Yesterday for instance I was struggling to beat Russia on Deity with my warriors and archers, but once I got my first horseman, the horseman and vampire alone were enough to beat down the remainder of Russia's cities without the help of the warriors I'd relied on up until then.
Vampire castles are also great, so there's that.
The real OP secret society is Void Singers though, and the real kicker is the 20% of faith converted to science, culture and gold starting from the Medieval Era.
Even if you don't have lots of faith production from Holy Sites etc., building monuments (which turn into Old God Obelisks), lets you get a very significant boost to these yields.
Heck, even for domination games, I value Void Singers as equally strong as the Vampires, simply because you get such a huge number of cities after a while all containing Obelisks, which secures your top spot for science and culture, nets you enough gold to keep up with army expenses, and gives you tons of faith which you can buy units with through the Grand Masters Chapel.
After playing with all the secret societes for a while now, I'd personally rate the Owls as one of the weakest tbh, considering the competition.
And yes, that includes the Hermetic Order which are also pretty bad.
Void Singers on the other hand are great regardless of what victory you might pursue.
Greetings!
I have not tried the others at all.
Try out the Void Singers, and you probably won't look back.
I tried Owls once, and immediately regretted my decision when the Medieval Era hit, whishing I'd just taken Void Singers instead.
That being said, I wish they nerfed Void Singers so that there was more of a choice, because as of now I find myself defaulting to Void Singers about 80% of the time (in all my peaceful games, and about half of my domination games).