I don't have one. I mean, there are definitely cover styles that repel me as a potential reader—ones that feature shirtless men, teen girls in fancy dresses, anime characters, etc.—but that's because I don't read those genres and those covers are completely successful in communicating that to me as a potential reader. In other words, they're good covers.

I suppose, then, the covers I dislike are covers that misrepresent the novel, intentionally or not. That's not a style, but it's as close as I'll get.

On the other hand, my favorite style is probably anything pattern-based. Think Coralie Bickford-Smith's gorgeous Penguin covers, or the original Barbara Plume hardback designs. I find them visually more interesting then a more traditional picture, and it takes an incredible amount of nuance to hint at a novel's tone, themes, etc. when working in a very abstract style.