Since it will only be 2 days and you have seen Kauai, I would opt for Molokai. Two days is enought to get a feel for the place, but not enough to really experience it. I suggest people go to Molokai for at least 3 days and spend part of the time reflecting on the simplicity.
Hawaii would take at least a week to get a good feeling for the island because of its size.
I personally, am not a Maui kinda guy. Oh it was beautiful, but it was more like Florida to me.
Still, visit every island you can for as long as you can, and then YOU decide if you want to visit again or move there :-)