For the most part, I like all the covers, but if I had to choose a least favorite (from the original covers),  it would be Scarlet. I feel like they had the opportunity to make the cover much better, instead of just a red cape.

But if you want to know about the redesigned covers, I would probably say Fairest, because the cover itself can make the story confusing. In the book, for those of you who don't know, Lunars (people who live on the moon, now called Luna) can manipulate the bioelectricity of others, or make them see things that aren't real or control them, making them feel things they wouldn't normally feel or do things they wouldn't normally do. This is called a 'glamor'. So, Lunars can make themselves seem more attractive than they actually are. Mirrors and cameras do not reflect the glamour, but rather, the person's true self. In this book cover, it looks like the opposite. It looks like Queen Levana is not wearing her glamour, but the mirror is reflecting how her glamour looks, which can make it confusing for new readers.

Sorry if I'm taking Fairest a bit too in-depth, but that's just why it's not my favorite cover.