100% cotton, comfortable cut underwear--find the best type for your body shape, rise, etc. Getting undies with the elastic bands sewn into the underwear and not exposed can ease on chafing. Make sure you actually buy your correct size. You don't want to be like those women who used to buy smaller sized shoes because they couldn't stand to have feet that were too big, right?
What are your jeans and pants like as well. If they are too tight you can't really blame the underwear.
Going commando sure won't help if comfortable underwear chafes or irritates. Seriously, don't try that.
Consider what kind of cleanser you use down there and on your legs, etc. Is it drying or a moisturizing, gentle body wash, etc...