Yes, I voted Velious, and no Im not a "fanboi" lol. Velious had some of the best quests with the armor quests, Coldain Ring/Shawl, the Tunare Quests in PoG, the miriad of cool stuff from Skyshrine and Kael. It had an incredible storyline as well, and gives the person who follows lore and factions an actual reason to do all that xp grinding. Velious textures were an superb graphic development when they came. Was a very deep expansion on many levels. No shortage of raiding options, questing options, or just grouping options. The zones were massive and designed to hold lots of people. It was designed with both high end players at the time as well as mid level players, and has been the most balanced expansion of that nature ever.
As far as cool zone factor mentioned in Magnu's post, there was Crystal Caverns (who expected a city to be at the bottom of an orc mine their first time there?), Velk's Labyrinth (very cool zone layout, no levi with ice slides, invis mobs, a raid mob or 3), Dragon Necropolis (insanely cool zone with very cool lore involved), Siren's Grotto, the list goes on. Almost every zone in Velious has something new and different about it. As far as extraneous rewards, ala LoY's cartography system and dyes or the task system from GoD/Omens, nah, there wasnt anything that made the game easier really, was just a bunch of challenging content with a story line and quests for everyone =).
I think Luclin gets a close second, aside from the horrible taste that all those patching downtimes left in my mouth Luclin was a well designed and very complex expansion as well. Though I wish it was the moon and aliens, but oh well....