So my favourite covers have always come from all over the place. For me, the best covers are the ones that drop a few mysterious and wonderful hints, rather than the ones that are trying to tell as much about the book as possible. Actually, I believe in 'show, don't tell' with book covers a bit more than I do with writing. (There's a whole other post in that, actually. Which I'm not going to write now. Remind me sometime, ok?)
Part of the reason I own most of Sarah Dessen's books is because for me they're the reading equivalent of high quality chocolate. When I'm depressed and need cheering up, when I'm unwell, when I'm longing for something familiar, I go back to her books over and over. They make me happy. But the other reason why I own them is because I love the UK covers so much. Deeply, deeply love them. I love the overall look, how they're a bit sixties in the best way possible, I love their colour scheme, I love how the artist has taken little details from in the books and incorporated them. Love love love. It's impossible not to buy books when their covers are this beautiful.
I also love covers when they're moody and dramatic. You know, like a thunderstorm in the middle of winter.
And also, whimsical covers are awesomeness. I really need to read this one, actually.
Find out about more beautiful covers at YA highway!
ETA: I forgot to mention my favourite NZ cover! It's a wonderful book too.