My favorites over the years:
Victorian England
I only got off my ass and took a look at this zone this week, and by god it's lovely. Well written, well designed, and some neat ideas implemented into it. I also just love the whole "culture" and atmosphere of the zone, which really sucks me in. This zone, to me, is one of the zones that just proves the astounding rise in "building" that 4d has seen over years and years. Always there are new zones that capture the mind and seem to draw you in even better than the last zone. It's amazing. Standards of building are rising, and it's wonderful to see. I also love the fact that VE was just.. easy. Not in the fact the quests were too easy or something, but it was easy reading. There were no silly tricks or ploys, everything was there right before you, if you only just used your BRAIN. It forced you to think, but when you found the solution, it was never something stupid or unrealistic.
Egypt
For some reason I have a passion for Egypt RL, and when this zone came in, it was my favorite for many many years. I used to know it like the back of my hand, not so much anymore though - sadly. Again though, beautiful atmosphere and ideas. And by Egypt, I mean the entire area down there including VOTK. I don't want to say too much about Egypt, for fear of giving anything away. Check it out, honestly.
Seraphine's Zone (What the hell was the name of it again?)
Oh, right, Enchanted lands. I've always referred to it as just "Sera's zone". Again, yet another zone that raised the standard of building a bit. She provided magic in her building, as well as darkness, an abundance of exploration, atmosphere and culture. An astounding zone with fantastic ideas and beautiful quests. And also, a lot of pretty things
Those are some of my top favorites, and it all comes down to RP, atmosphere, and what I like personally (I love egypt, and I sometimes wish I'd lived in old english times. They had such gorgeous corsets and gowns...)