First and most important backcountry lens is a telephoto zoom. On an APS-C crop I really like my 50-135mm f/2.8 for this purpose (though I shoot photo not video). I think the 50-100mm range is very important to have on a crop sensor, though a 70-200mm style lens could also work really well.
Lack of zoom (e.g. 50mm f/1.8) can be frustrating because 99% of the time you can't zoom with your feet when in the backcountry.
And the second thing would be a wide angle lens to shoot things close up. Here you could choose a prime over a zoom. I'd say something between 16mm and 20mm is ideal for this purpose. Most people here will have a 11-16mm set to 16mm or a 17-50mm set to 17mm.