If i had drop down to one gun it would be my 16/7x65r Ferlach combination gun.
It handles beautifully, and whilst only a single shot, I shoot it really well and its accounted for phaesant, woodcock, duck and squirrels. On clays I can hold my own on the driven grouse and high phaesant targets. And with the rifle barrel I have plenty of foxes, roe, red and wild boar out to well over 200 yards.
Indeed I think it is the perfect all round gun. You could even complete a McNab with it. And yes I have seen a Salmon taken using a Grand Prix fly - waited till one jumped at the falls pool!