I hate to be that gut...but it's el mapa. One of those fun words ending in a that's masculine just to screw with foreigners.
EL mapa??? Well that's just preposterous. In addition to the fact that it goes against all possible grammatical logic, la mapa sounds smoother AND is easier to say. I'm really quite incensed.
Haha actually now that I think about it I'm pretty sure we've had this exact discussion before.