One for now, or one for all time...? That makes a BIG difference!
Right now, it would be my 1200 GS (or a newer version). My kids like to ride with me on the GS and go exploring. That's major quality time. After they are grown and gone... Hmmm... I still like the GS for the type of riding I most often GET to do versus the kind of riding I'd like to do more often.
Like Vinny, if I lived somewhere with more mountains, it would be a KTM 500 EXC (or the Husky version).
Really, I like having two bikes, my 1200 GS, and my 530 EXC for those times when I can get away to areas with more mountains.