According to Aya, Suika is the most powerful. But Yuka and Yukari are also among the strongest. And of course, Reimu and Marisa. So there's no actual answer to that question.
The most powerful characters that we've met so far in the games would most likely be either Suika or Yukari (or possibly even Yuugi, who was just messing around with you during Subterranean Animism and is far more powerful than her Stage 3 boss status would suggest). There's also Kanako and Suwako, who are gods. If I understand Shinto correctly, gods are not necessarily supreme beings or automatically more powerful than youkai or oni; however, these two enough power to teleport their entire shrine, as well as to give Okuu nuclear powers, so they're clearly up there on the Gensokyo power scale.
As for Reimu and Marisa...they're both pretty powerful, but really no match for a lot of the Stage 5, 6, and EX characters. The reason Reimu and Marisa are able to beat them is because they're masters at danmaku play.
The entire purpose of the Spell Card system is to come up with supremely challenging - but not impossible, and most importantly, non-fatal - attacks. They're meant to be tests of skill more than power.
Out and out, Yukari and Reimu are at the top, with Yukari at the top and Reimu in second. The reason for this is thanks to their respective powers. Yukari's manipulation of boundaries of any sort means that she can literally do anything at all, since all she has to do is flip the boundary of fiction and reality and Cthulhu is real. Flip of boundary of Master and Servant, and now Cthulhu is her servant. She wouldn't even need to go that far though, when she could just make you not exist at all in the same way. It's simply because of this power that she's above Reimu though, as Reimu's ability to 'float' allows her to literally float away from reality and become utterly invincible. Even though ZUN's comment that with this ability on her side, Reimu would be unbeatable by any means, Yukari likely could still affect her by reversing the boundaries of reality on Reimu.
Behind these two would be Suika and Yuugi tentatively tied, as Oni are the most powerful type of youkai and Yuugi's power isn't clearly defined enough to be sure whether or not it's better overall than Suika's density manipulation. Everyone else is subjective.
first sorry for my bad english.
and what the question about the powerfull? power level or somewhat if she fight she can beat everyone? Every character has her own skill ability and characteristic, so like fighting game. anybody can beaten anybody.
if you keep about powerlevel, just following your self intrepentasion (your head is your fandom, and touhou is very free about it), or just following ZUN tier in StB and Double Spoiler.
As for me, i think many fans always overtiering Yukari. if it's because her ability don't forget about PC-98 chara. Yuka can freely go to another world (to to gensokyo, muenzuka (LLS stage 5), another dream world (LLS stage 6), and somewhat future wolrd in kioh gyoku). mima has her own world in SoEW. and Elly and Kana is the guard of dimension border. and her ability is supporting danmaku, not like other who can't use their ability into max potential because of the spellcard rule.
The character who interest me about the power level is Kanako, Suwako, and the Lunarian. Suwako in soku give me indication if Yukari can't use her border in moriya shrine. the gods has their power according the faith they has. in MoF they lack of it (about dissaperaring of gods, maybe they still exist here just because only from sanae personal faith). and after MoF they have faith from all of the youkai mountain.
For Sanae, Zun put her in same level as Reimu and Marisa, if Zun just put her direcly in UFO i think the fans still place her level under that 2 main chara of this series, but Zun make her beat Reimu, Okuu, and Suwako in soku story mode.
About the oni, they sure strong, it's even lol wud for me why zun take them place in lv 6 in DS. but when i see the character in lv 5 and 7. and think for a while it's come to make sense. maybe zun wan't to take they place same as Byakuren underlings. especialy for Toramaru shou (byakuren team = journey to the west team). when ufo was release i remember someone in poolshmer say abous Bishamonten 4 devas. if yama no shitten douji is basicaly from 4 bishamounten devas, so it's makes sense to place shou bellow the oni.
Powerlevel actually is an interesting topic to talk, but very potentialy can make Troll and Rage.... and i forget to answer the question who the stronnger here (for me) for me is no one.
btw i just remember if Reimu, Marisa, and even Yukari can't beat Watatsuki sister in combat, and they even not beating Eirin, Kaguya, Keine, and Mokou in 1 vs 1 battle.
Well, Reimu and Marisa fought half-heartedly against Yorihime, and Yukari threw the fight against Toyohime. However, Yukari did say that Gensokyo couldn't match the moon in terms of power. And while neither Toyohime nor Yorihime show up in any of the games, their teacher - Erin - does, and she's displayed significant power. So yeah, I guess I'm kind of dumb for forgetting to mention Erin in the top-tier characters. (I don't include Kaguya because ZUN claimed that Erin is more powerful than Kaguya; nor do I include Mokou because she's supposed to be on par with Kaguya.)
I will also agree that to say Yukari can do anything is vastly overrating her power. If her boundry manipulation truly made her all-powerful, it's doubtful she would have created Gensokyo; after all, Gensokyo is meant to be something of a youkai nature preserve to protect them from humans. Yukari can manipulate boundries and gaps; she doesn't have absolute control over all boundries and gaps.
It should be noted that Perfect Memento In Strict Sense is not meant to have true or even very accurate information on the youkai it chronicles. It goes off of what both Akyu thinks the case is, as well as the subjects themselves coming to her and trying to influence what's written about them - and, needless to say, a lot of them boast or try to make their entries more impressive. Yukari, being the devious and manipulative person she is, probably thought it'd be fun to imply that she's basically omnipotent.
There's also the short story in Seasonal Dream Vision wherein Yukari feels the presence of someone powerful and tells Yuyuko that they're no match for this unnamed person, and they'd better leave. My guess is that it's Sikieki, who, as a divine judge, would be extremely strong against a youkai.
Despite all of this, though, Yukari is still incredibly powerful, and still a contender for "most powerful in Gensokyo." There may be others who match or surpass her in certain areas, or can defeat her because they have divine power that she's weak against, but overall, she's probably the most powerful character we've met so far in the games. Her power and influence shows up time and time again in the games, short stories, and manga.