Technically, the first thing Cheetah did was conquer Paradise Island by stealing Hippolyta's girdle which is pretty noteworthy, especially considering this was Priscilla Rich and not the god-empowered Barbi Ann.
I think this is an important statement. I can't see Diana developing any level of hatred for someone like the one that Batman feels for Joker. I don't think she's capable of it.
Five years ago, I definitely would have put Ares up there, but now? I don't think he's even close to being in the running for her arch anymore. He's had virtually no storylines where he's a villain in the past decade, and his comic history before that is very spotty. Not to mention he's often cited as the weakest part of the first WW film (which I agree with). I like Ares when done well, but he's been done well FAR less than even Cheetah or Circe, who have both had some good stories in the last 5 years.
Honestly, I don't really see Circe as much of a contender either though. Cheetah has some really bad stories under her belt, but that doesn't change the fact that she'll always be the villain DC shows Diana pitted against on promotional materials. Even if her reception in WW2 wasn't great.