Hi, Paul! That's an interesting question, and I don't think there's been either a specific discussion thread, or anything definitive by Paul ever posted about it.
My first (very subjective, and by no means "official") thought is that it's too simple a question... or, rather, that it assumes too simple a sort of answer. The key word here is "levels", which sorta implies "more" or "less" of just one sort of "power".
From what I think we've seen, the answer in Wapsiverse is more complicated than that. There's power of quite a few types, and influence / clout, and knowledge (which is, of course, power of a sort), and wisdom. The various characters you've mentioned (and the others) each have their own interesting "mix" of these things, and I don't think we can always rank one as "more powerful" than another in a clear way. There's probably not just a single-axis "hierarchy", but a complex web of power-relationships of many sorts. Think of it as a cat's cradle of strings, in at least 15 dimensions, and probably fractal as well... that's the sort of picture I see when I look at the Wapsiverse.
The traditional gods and titans are very powerful in their own way, and immortal or nearly so... but they're not without their limitations. The Vimana shield set up by the Golem Girls has been able prevent even the gods from traveling freely to Earth, and Bia (a titan, and the very personification of Force itself) could be derp-punched right through a solid stone wall by low-sphinx-on-the-totem-pole Phix, when Phix was in the Library. Bia is clearly very powerful on a physical level, apparently has had a neurotic obsession with Fixing Things for many thousands of years, was very much a manipulative sociopath, and seems to have become a lot gentler and saner since her father turned her back into a child.
Monica is a "sub-Titan" (so, presumably not as much raw power as Bia and the other Titans), and her powers are somewhat specialized (poiting and Lanthian glyph-reading command powers over the Golems, regenerative immortality, the ability to see a person's deepest fear, and the ability to walk to the land of the dead and to interact with demons). We haven't seen her spit fire or make the planet turn backwards (although she does poot in rainbow colors, I believe).
Shelly... the immortality and strength of a sphinx, the wisdom of extreme age, and the power of her Titan ancestry... but she's still scared to death of spiders. Lately she seems to have settled into a somewhat traditional role of "protector sphinx" and doesn't seem to be ambitious beyond that... but Bia believes that Shelly has a Destiny. Probably still a lot to be learned about Shelly, I'd guess.
If Shelly and Monica should fight... I'm not sure that Monica could kill Shelly, and while Shelly could very probably kill Monica any number of times Monica wouldn't stay dead. So, which of the two is more powerful? Beats me. Not an easy question.
The Golem Girls... from what Shelly has said they're probably among the most powerful of human-descended paranormals, and any of them would be quite capable of turning our planet into a charred pile of rubble... but Monica can (probably) lock them down and put them to sleep with a few words, and they can be manipulated by others (the Nu Gui took control of Brandi, and tricked Acacia into suspension).
Atsali... Siren/Incubus, she has the compulsion-power of her heritage, and she packs quite a combat-punch as well (surprising even herself). It's TBD what more she has lurking "in the wings" as she grows into adulthood.
The Fae... apparently both powerful and dangerous by para standards, quite amoral, but they do have their own specific constraints (honoring the Blackthorn so much they can't even defend themselves against a direct attack by one) and they're definitely not immortal.
Pickle... she's a partially-matured bio-engineered weapon, certainly with a lot of power available to her now (enough to kill Fae and many other paranormals) and maybe more to come... we don't know. I'm sure Paul will spring this on us in his usual inimitable bootlegger-turn fashion one of these days
Then, there are the manipulations of the demons, the sphinxes, the immortal Lanthian politicians... a mix of "power" and "influence" and "clout".
So - what you ask is not easily answered, I think.(Except, of course, for the lingerie ninjas. There's no doubt they're the Secret Masters of the Wapsiverse. Sorry to give away your secrets, Paul! )
And even they take orders from Conscience.