I agree that over about 16x you start to get into a little trouble, paticularly on hot days, with the heat waves. Fixed powers work best for me at fixed distances. I pesonally like 12x for my bench and varmint shooting. I've used Leupold, Redfield, and Weaver 4x12 variables with good success. My old Unertl 15x Ultra Varmint works well, but is a bit cumbersome anymore,and sees only limited use.
Until my eyesight gets much worse (I need reading glasses anymore), I intend to use my 12x's. Several years ago, I shot a blackbird at a measured 297 yds. with a .243 and 12x optics, while groundhog hunting. I bet my buddy a cold one that I could kill it. He said "Yer on!" He bought.