King's Landing really was impressive, but my heart belongs to this painstaking reconstruction of the Citadel Presidium, from the Mass Effect series. The eventual scale of it boggles the mind: the creator has a full gallery of clearly recognizable images and still, at the time, claims to be only 20% done. Now all it needs is blocky space pigeons and the effect will be complete.
Mass Effect Presidium [20% done] I need skilled builders ! [Planet Minecraft, via IGN]