Besides Lionheart and Phoenix, two talented cartographers in their own right that I like, I'll add my opinion on some other favorites as well:
NinjaNic was one of the best mapmakers from the previous generation. He made the Continents of Competition and several others that culminated in the "Particular Geographic Region" that summed up his work pretty well.
Chaos created five archipelagos of Battle Islands, the fifth of which remains a popular battle template in the ladder. Many of his maps were also pretty popular early on.
Among the more recent mapmakers, Absolutely Ethan has done a tremendous job with commissions and is championing the 3D map system.