All stiffness scales are not created equal. If i was to have a scale of 1-5, 1 being a tennis shoe, 5 being a 2x4 mountain boot, I would personally look at something in the 2.5-3 flex rating range. I run Salewa Mtn Trainers, and i would say they're about the perfect 2.5 flex for me. I think boot flex is very subjective also. Some people like a stiffer sole, but a soft upper like the Crispi Thor. The Thor might technically be more stiff than a crispi Nevada at a flex 3, but in my experience the nevada feels more stiff due to being a full leather boot. Go with the most well rounded boot that still lets your feet flex and bend, but provides enough support to where you don't feel like you'll hurt your feet. If you don't have weak ankles and have tough feet, get something like a lapponia.