Alabasta was a good arc. Not only did we get rid of a Shichibukai, but Luffy's bounty increased, we got a new crewmate, and we had some wicked awesome fight scenes.
In Skypea, we got SOME info on the history, some new weopons, and we took out some random guy with no apparent significance to the story who had a major God complex.
Skypea added little to the overall story. It got downright dull at some points. Let's just hope they don't make the same mistake with Fishman Island.