I will chime in with everyone else who has said a "blend" or "what ever I have the most of."
Because usually if you are hiking with other people they will want a swig. And sadly I am never hiking with single malt fans. I am just happy to have them ask for a nip.
A year ago I backpacked through Europe and would just buy a new bottle when my flask ran empty. Mostly I remember having Ballantine's Finest (not that great), Jack Daniels (I am from Tennessee), and Macallan CS (It seemed to last longer and go better with the Havana cigars we enjoyed on the trip).
So ya, Never put an Islay in my flask even though the malts from that island are my current drink of choice.