J.R.R. Tolkien. From the main Middle Earth map, to the way moon-letters were introduced in the hobbit, to the large foldout in the Silmarillion. To this day the stylistic elements of those maps are copied across in other works.
Bethesda Software has also done some good work on making excellent maps detailing the regions of Tamriel. The newest, Skyrim, has even made guide publishers like Brady create very large interactive maps (that will absolutely crash browsers on powerful machines). Again, you can look at one of their maps and almost immediately know where it came from.
I'll agree with most people here and say that the folding map in Lord of the Rings (and the works that were published later)
are probably some -if not the- most iconic maps in the fantasy genre. I can certainly remember spening a lot of time as a young kid searching for
the current location ofthe characters. I even ahd a poster of middle earth right above my bed ;). Apart from that I guess most other maps from well known fantasy Series like "wheel of time" or "a song of ice and fire" share some popularity.
The maps from various Dungeon and Dragon Settings have also been mentioned. In addition maps are an integral part of the gameplay in most games, especially Rpgs (I think Baldurs Gate has not yet been mentioned) and Strategy games, though I guess thats pretty self-explanatory. Another interesting example might be Minecraft were maps can be created within the game, but I don't know if that counts as an RPG.
Personally, the most memorable map to me is the map of Britannia from the Ultima games... almost iconic.
Second is the map of the Four Lands from the Shannara series... it always reminded me of a cool D&D map
I would say the map from 7th Sea, an RPG created by John Wick and published by AEG. It was drawn by Craig Zipse. Here's a fan site that features a couple versions of it... http://project7thsea.electronicescape.com/?page_id=46
Meanwhile, I own the original. It's framed and on the wall of my gaming room. :)