Best MOS? If you could re-class to a different specialty, would you and why?

Apache Pilot.  No explanation needed.

Good question Ben.  I served in the USMC as an infantry officer (0302) during peacetime from '83 to '88.  Looking back I would not change a thing.  I served with some great Marines and have many fond memories of what we accomplished together.  the passage of time has helped mellow my memories of the not so pleasant experiences and I see them more clearly.  Thanks for asking.

Without a doubt, I would join the Signal corps, for no other reason that it would have directly prepared me with years of experience for my current job, potentially even resulting in a higher starting wage. I know, not a very exciting answer.