I'm thinking about aesthetics, specifically, not lore or gameplay. Though it might be controversial, I would pick Kaineng City.
One of the biggest changes from GW1 to GW2 is the new game's ability implement verticality. I just think it would be awesome to travel from roof to roof among giant structures, across this sprawling megacity that stretches to the horizon, while looking down towards to undercity deep bellow. Or working your way up from the cavernous sewers, for what seems like forever, until you finally reach sunlight. Imagine Draconis Mons type heights. It would be both intimidating and epic to explore. Plus flying a griffin between skyscrapers and under bridges would be fun.
Though the GW1 Kaineng was cool, if a little confusing, I think GW2's engine could really do the original concept art justice: https://wiki.guildwars.com/wiki/Kaineng_City