Shiver by Maggie Stiefvater.
Thoughts: Definitely think that in both 'Shiver' and UK book cover version (left) rules the day. It looks darker, more of a contrast and more eye catching if sitting on bookshelf (unless beside twilight lol).
The Replacement by Brenna Yovanoff
Thoughts: US (left) without a doubt a winner here. The UK cover looks so generic YA PR that it wouldn't catch my eye at all, while US cover just makes you want to read it!
Morganville Vampire Series by Rachel Caine
(e.g. Book 1: Glass houses)
Thoughts: Neither of these covers do anything amazing for me. But I do think that UK cover (right) appeals to me more. It definitely knows its market. I think the later covers on the UK side do improve, and are very pretty. The US cover (left) is edgy but just doesn't leap out at me.