guess it depends on the rider... for me, a GS would be an utter nightmare off road & i don't even like it on road. i like the idea of a ninja 300 motor in a capable chassis/suspension package. for me the closest thing to a great ADV bike out of the box is an XR650R. It's still on the heavy side for single track, but it does the wide open throttle boogie through the desert that i love so much.
otherwise... if i were building my version of an ADV bike... either a late suzuki RM... or KTM frame/suspension, wedge a ~450cc thumper in there... or maybe a big rotax 4 stroke...
current: '89 suzuki gsxr 750, '77 yamaha IT250, '67 harley xlh chopper, '59 harley xlch resto project, and mucho basket case projects
past: too many to list