Ah i see.. you are looking for the true bad girl of the series? Its debatable but i'd say no one is the true BAD bad girl.
- Reimu or Marisa would be possable Hero Antagonists. But even then Utsuho is the only Character that have had a goal which is unquestionablely evil (controling the world)
I dunno about that. Remilia blotting out the sun so she didn't have to bother with a parasol, or Yuyuko condemning an agricultural society to several additional months of winter so that she could try to make a cherry tree bloom, both seem pretty rotten.
Also, depending on how much of the PC-98 stuff is considered canon, Yuuka is pretty nasty. This is a character who said that "genocide is just another game; it doesn't matter if it's humans or Makai residents." And no character is portrayed as a vicious sadist in fanworks as much as Yuuka.
I also have to agree with the statement that Reimu and Marisa are both horrible. Reimu, in canon, is almost always a selfish, hateful bitch, and Marisa is an unapologetic thief. It says something that only youkai frequent the Hakurei shrine.
Despite all of that, I'd say the worst, hands-down, would be Tenshi. Tenshi destroyed the Hakurei Shrine and was going to destroy all of Gensokyo because she was bored. Then, on top of that, when tasked with rebuilding the shrine, she not only slacked off on the job, but she set it up so she could make the shrine hers. The only reason she didn't get away with it was because Yukari figured her out and didn't put up with her shit.
(On the flipside of this question, it's kind of ironic that the character that all the other characters figure is the worst, who everyone hates, the one who is never up to any good - Yukari - is possibly the most morally upright of the lot. When her friend Yuyuko requested help, she gave it. When she found out what provoked the moon-sealing incident, she welcomed the Eientei crew to Gensokyo. When Rinnosuke needed fuel, she gave it. I'm pretty sure her actions in Silent Sinner In Blue were meant to diffuse tension between the Lunar Capital and Gensokyo. When Tenshi attempted to infiltrate the Hakurei shrine, she thwarted the attempt.)
Alternately, I found a Youtube comment that reveals something I never really thought of. "All the problems since Touhou 10, all track back to Kanako's actions as the source. There for, she is the ultimate bitch of Touhou. Awesome!"