Like many questions, the answer is "It depends." It depends on the purpose of the map, how it will be used, how often it will be used, and probably a lot of other things. Since this is posted in the Board Games forum, I'm going to make these assumptions: The map is for a board game, it is a professional product that I'm going to spend money on, the game is to going to be played many times, and the map is the central prop for play. (Ticket to Ride, Small World, Risk, etc)

The qualities I think are most important for a board game map are:

Lays flat
Easy to read, even when upside-down
Spacious (not necessarily large, but having enough space for all of the pieces that will go on it)