I would not call it best bang for buck. It's a great lens, with a very good focal range for "everything" and is great for landscape, as well as everything in between. But I wouldn't call it the best for landscape, at least, for the cost.
I find most national parks like wide angle lenses, but, you always often want a telephoto for wildlife while there, as you're bound to see something at least. You may want to consider that, as well, unless you already have something. Since you're on aps-c (I assume since you're looking at the EF-S 15-85), a 200~250mm long end lens will do quite well and cost little.
Some lenses maybe to consider:
Rokinon 14mm F2.8 - Sharp, fully manual, inexpensive.
Rokinon 16mm F2 - Another lens that is a stop faster, worth looking at if you like wide field astrophotography. Fully manual.
Tokina 11-16 F2.8 II - Ultrawide with a great range, sharp lens, fast aperture, prone to flare if pointed at a light source. Takes filters.
Tokina 16-28 F2.8 PRO - Their latest ultrawide zoom, very sharp, fast aperture. Not as wide as the others, as it was meant for working on full frame as well as aps-c. Great focal range though.
Canon EF-S 10-22 - Ultrawide, very sharp, great control. If shooting landscape in the daylight this is probably the better lens to have on a crop. Takes filters. Only con is the slower apertures, but this doesn't matter for landscape, it only matters if you want fast aperture for night sky photography.