I have to think it is Huckabee at the moment. He was the earliest to get out and back Trump.
Rubio probably number two to add a little more youth and maybe go for some more Latino voters.
Christie/Giuliani/Newt/Palin - These seem to be the inner circle. Probably all will have cabinet positions.
My guess is that Trump already has his VP lined up, and he will announce the name when he feels the time is ripe.
Just my two cents.
Huckabee is unlikable and has zero positives.