Croatoa's probably my favorite zone. Even if I'm not teaming there (I rarely solo), I usually take the time to go badge hunt just to enjoy the scenery. I played red-side mostly back in the day and every time I set foot over there and zip down some Sharkhead side street I'm all "oh yeah, there's that one place, nice seeing it again" but of course it's tough to get teams red-side, so I'm not there even half as much as I'd like to be (probably time to roll another Mastermind I can solo there decently).
I often forget about all the off-to-the-side blue-side zones like Striga, so whenever I get on a team going to those places I end up feeling silly having to ask "er... how do I get there?" like a n00b. Never had that problem red-side!
Ultimately, I think I like the zones of either side that have some natural areas to them, not sure why. Croatoa's a good example of that, here you're in town, then suddenly a pumpkin patch and then in a mysterious woods and now some hills with... uh wooden platforms for some reason.