I find on top works well because I'm in control so I don't let it get uncomfortable but it hurts my legs really quickly.
From behind is sometimes ok but I still find it a little uncomfortable at times. My fave is, if he has strong arms, him on top but he holds himself mostly off your body and has his legs out quite straight, like he's doing push-ups. Then if you bend your knees you have to do most of the work but it's still fairly deep but somehow angled away from the cervix so it's not at all uncomfortable and I find kills my OH every time!