The best map in GW2 for me are the expansions' maps, so Hot and Pof and also some of the LW4 maps. These maps, first of all, fit into the context of the story and have a coherent theme. They have depth, lots of minor details that make it come alive. From NPCs and their dialogues, events and meta events it all fits into the theme and the story of the map. So I would say a good map has a living, believable ecosystem with lots of minor details that fall into the theme of the map.
They also have a lot of secrets. Even Elon Riverlands that feels very flat and boring at first have a lot of secret places to explore. Also good maps should have some really tough enemies but also places where you can chill.
These maps are also beautiful.
Well, I'm not doing core game justice because many of the core maps are on par with expansions. Expansions just stepped up on the aesthetics and the enemies in core maps are too much of a pushover nowadays.