After yet another Impel Down re-read I have to say it's become one of the lesser pre-timeskip arcs over the years in my eyes. It almost entirely coasts off the returns of already established characters, which is fine but not really my cup of tea.
The opposite trend has affected Thriller Bark imo. A very solid adventure through n' through.
Sabaody > Skypiea > Alabasta > W7 > Thriller Bark is ~ my top 5 atm.