I tried many pants at REI - Sahara, ExOfficio, North Face, etc., and found that the best hands down were the Mountain Hardwear Convertible Pack pants. There are no bunched elastic parts to rub your waist under your hipbelt.
Even so, I've found that my pack (Gregory Z55) tends to push the back of my waistband down towards my hips. Does anyone else have this problem? (Maybe the pants are slightly loose on the waist). Do you guys wear your pants high or low, below your hipbelt? I certainly don't try to wear my pants low on purpose, as seems to be the fashion statement these days. I find a higher waistband more comfortable myself.