Ok, I'll throw my opinion out there.
It's hard to meet this criteria. If little Johnny weighs 90 pounds and Grandpa Rick is pushing 300. A single board to fit that bill is going to be hard to find.
Personally, I like the larger surfstyle boards. I feel they are the most stable and easiest to learn on. I haven't ridden the new Trick Boardz Mojo, but is was designed to be a cruiser.
The Inland Blue can handle folks from 140 to 270 and it will fit in a standard board rack. There are offerings from Surftech, Shred Stixx, Shoreline, Awakesurf, XBC (did I forget anyone?
) in the @ 5'8" range that will allow virtually any adult and all but the smallest kids to get up and free surf. These boards require a special rack, or to be put away, however once on the boat.
The Hyperlite Landlock at 5'11" isn't bad for this situation either.
To be honest, at this size and rider ability, most of those boards ride the same and offer about the same level of "ease" in learning. IMO there isn't a clear hands down "best board" in this category. It will probably come down to some other factor, such as availability, price or convenience out on the boat.