Surprised no one has said Minmus yet. The close proximity to Kerbin means it takes much less Delta-V than most other bodies, it's low gravity and salt and/or mint ice cream lakes make it good for large single piece base structures along with the fact there are no oceans making it easy to navigate without relying heavily on fuel like you would need for long distance journeys on Laythe, plus, it still gives you the second best view of Kerbin out of the options given to us.