Best: FedEx. I love the way the purple drains from the tail into the body. It's the same reason why I love Qantas. Seeing the red tail drain into the body is cool. Now, graphically I love Seattle and seeing all the smiling Eskimo's lined up on the Alaska Airlines tails is pretty cool, I just don't care for the balance of that livery.
Worst: I like some of the newer America West looks with the graphic AW treatment, but have always cringed at the AW logo itself. The only airline that hasn't 'updated' itself since it's launch. Also worst... the UPS cargo planes that they experimented with flying limited passenger loads a few years back. Brown is not a very pleasant color for a flying cigar tube.