Reimu is running the barrier, but we see Yukari repairing the barrier once. It consists of allowing some fairies to live in a tree. So yeah. And the Dragon God created Gensokyo itself.
Looking at the official comics, Reimu's basically the janitor for it. She just keeps it functional, clears away the riffraff, but doesn't really get how it works. Heck, she damages it by accident every so often, I guess shes really just the focus for the barrier.
As for Kaguya, as seen in Imperishable Night, shes easier than Eirin....but only when shes just showing off her Impossible Requests. The End of Imperishable Night cards are the ones using her manipulation of eternity(ending the spell of Imperishable Night in a little over a minute), and THOSE are friggin impossible. So I dare say she can go up the rankings a bit, her bling isn't that special as final bosses go.
Reimu's hax though, is that in addition to her Hakurei Miko powers over the barrier, yin yang orbs and natural talent(I think this is the flying miko and fantasy heaven origins), she can also channel apparently any number of japan's gods to do whats needed. If she bothered.
So, put into tiers I'd suggest the following Tiers:
Tier 1: God/Ruler/Creator of a realm of existence. The Dragon, Shinki, Shikieiki as examples. They're beyond the pale. Yukari and Yuyuko might fall into the 1.5 range, they can be said to have a realm, but aren't exactly in control.
Tier 2: Full out reality warpers. Yukari and Suika. To a lesser degree, Reimu(when really rockin ALL her mikopowers), Sanae and the Watatsuki sisters, followed by Ex-Keine(note, nobody could find the hidden village except Yukari, and thats because she could see the border), Kaguya, Eirin and Remilia. These 2.5s either have a narrower range of manipulation, limitations on their use, uncertain effects or they just aren't using their full effort. Also, any Magician, with time and prep, can temporarily achieve effects of this level via some spell anyway.
Tier 3: Person of Mass Destruction. Pretty much the bulk of the cast. Flandre and Yuuka sit at the top of this, they'd be higher, if they could do anything except fight. Marisa, Alice, Mokou(on the high end due to immortality), Okuu, Ran, Patchouli, Sakuya, Aya, etc, etc. This is primarily a measure of sheer brawn, they could beat higher tiers if it comes to a contest of raw power, but they won't be editing reality. Most of these are Stage 4 and above, but theres exceptions(Alice is running her whole Stage single handedly, plus not fighting seriously, "Well, perhaps I shall partake of your small spring as well?" Hur Hur). Generally if its been a main character it's at least here. Cirno might have to be a 3.5 in her later incarnations by that standard though.
Tier 4: Dangerous to humans. Generally hazardous night predators...if you were an ordinary human. Mystia, Letty, Cirno, Chen(though if I'm not mistaken, Chen could be 3.5, shes a full blown spellcaster, but she wasn't fighting that hard) etc. Any main character generally finds them to be little more than a speedbump though.
Tier 5: Laughingstock. Most fairies and probably Stage 1 Bosses. They'd pose a threat to a human, but any adventurer types probably won't have much difficulty in getting rid of them. More because of being dumb(Unnamed Fairies) and incompetent(Rumia, Miss I Can't See Where I'm Going) though.