I guess it's not so different as the pro level. I think the "easiest" is runningback by far as Outlaw said (understanding protection and recognizing blitzes are the most critical tasks), then maybe wide-receiver and corner, but in both positions you'll have to understand the route running thing and the coverage recognizing respectively. IMO at the CB position, if you're gifted, you'll be able to impose your physical condition and your will, for sure. I'm not sold on this for wide-outs, where you'll have to understand route running and how to make you wide open. Well, the most difficult is QB by far...
RTR,