The Harbinger mission's setting is incredibly well done, joining Europa as a top-notch setting.
I'm not going to debate the pros or cons of the mission, but I had to acknowledge that the whole crumbling dam setting was so well done, such amazing artwork, especially the main transformer room. This is a location that cries out to have secret doors or places that need to be discovered.
Add this to Europa as a setting I'll never get tired of exploring. Again, say what you want about the activities on Europa, but the whole thing is incredibly well done artistically. The snowy vistas, the abandoned cityscape, the clean and colorful lost Bray labs, just top-notch art design.
Bungie has always had great art talent designing their levels and planets. I wonder if I would have played this game as much as I have if it didn't look so good.