Ok, this week there are two different quesitons, so I'll edit and answer both!
Q: Best cover? What is the best cover of a book that you've read and loved?
Technically I haven't finished this one yet, but I'd say Stormdancer by Jay Kristoff. Both the UK and US additions are beautiful.
Q: Best cover? What is the best cover of a book that you've read and didn't like?
This is harder than the previous question. Books that I didn't like, I also didn't like the cover! lol. The Hunt by Andrew Fukuda - I didn't hate the story, but it's the best answer I can give if you judge it by the US cover.