Map 1-9 (Field Map)
Map 10-13 (Lava Cave)
Map 14-17 (Thunder Cave)
Map 18 (Unown Map)
This Map cant be found in the maps list though.It has the Fewest number of users most of the times.
but you can enter it by going to http://www.delugerpg.com/map/18 or Map 3 and then enter