Generally, the main mission (Garland Square) should adjust difficulty to according to player skill, but if you are new to the game or have not played the arcade version, here is the recommended order in which you should select the zones, with the least challenging first, and most challenging last. This also seems to be the main default order that the game chooses on selection timeout:

Drycreek Plaza (Easy)
Garland Park (Medium)
Garland Technology Center (Hard)