I'm the last *ing person to answer this - not that it won't stop me. Also, I think his perspective is as a longtime windsurfer. A long time kiter would, of course, have a different take.

I'm going to offer several options because different strokes, etc.

0-5 SUP, longboard, efoil
6-12 Longboard, 7-8m2 wingfoil, 7.5-9.0 windfoil, kitefoil
12-20 Wingfoil, Windfoil, kitefoil
20+ shortboard (80-100l. depending on body size), wingfoil, windfoil, kiteboard

Notes: Some would say the longboard would cover the entire range and I'm not going to dispute that. It's not my flavor of ice cream though in 10 when I can be absolutely lit on my windfoil gear. That different strokes thing again ...

At the margins, skill will matter. Any kind of foiling in 6-9 will require skill to be truly rewarding: pumping, making consistent transitions, etc or you face periods of lulls waiting for a puff. Of course, a very good way to acquire those skills is to be out in those conditions even if you lack the skills. For kiting at the margins, both light and heavy air, doubly so as the local environment can be very challenging. A ATL kiting FB mod even recently told a guy not to even bother kiting here (which I disagree with).

Lastly, since my domain is more windfoiling, in the "less is more" theme.
6-20+ high aspect windfoil/big sail (for me, 8.0)
15-27 mid aspect windfoil/4.7
20-35 mid aspect windfoil/3.5

Alternatives for the first spot
10-22+ 7.0 with high aspect foil
11-22 6.0 or 5.4 with mid-aspect foil
0-10 is taken up by bike riding or hanging on a keelboat

What say others?