Worst: Heathrow, Heathrow and Heathrow for the reasons listed above. "Worst" not only includes any airport where a bus is involved, but any where you have no alternative but to get dumped outsode the "secure" area and thus have to go through "security" again. See Heathrow...OK, I'll stop ranting now.
Best- ZRH- we went through only once but were impressed. "Best" includes any airport where you can manage to stay inside the secure area when connecting.