The Mac 11/9 with a Lage upper would be my choice. One of my favorite shooting sub-guns.
My other choice (and the one I'd start with, if I were buying tomorrow) is a Sterling. Great gun, extremely reliable, super easy to shoot. Not super "modernizable" but still a really great gun.
When I finally settle in a place and start buying full autos, my three will be a Lage-equipped Mac, a Sterling, and a M712 Schnellfeuer.