Well, here are some favourite zones.
Alpha Centauri (Herbie is cute, but I also like the many (funny) scripts on the mobs which brings flavour to the zone. Eg, talking with His Arrogance Ai Poppy Googy, the Yalcans in Yalc. Possibly the funniest zone in the game) Also led me to read the Retief books by Keith Laumer
Victorian England (It really seems to have everything for everyone. Nuff said. And the fact that you don't need to be a T4 L50 to complete any quests there)
Midlands (I like the maze, as well as the ways of finding/getting the keys to the knothole and the manor room)