One of the people I asked for help in picking a cover for my book was J. Kingston Pierce who runs a marvelous blog called Killer Covers. He confirmed that the one we eventually chose had the greatest impact visually and in terms of giving you some idea what the story was about. I liked them all.
I don't look at book jackets all that carefully. But there is one series I like. I think it is a great idea to link books in a series as much as possible and yet still keep each one distinct. Here is my favorite: Rennie Airth's. Same set-up but different color fonts and different pictures. Jeff discusses this more fully on Killer Covers.
What is your favorite book jacket and what series has done such a good job in linking its jackets?