Nothing beats a bag that has a waist belt/clip for me. they usually double as a sling bag as well. It distributes the weight from waist down to be supported by the strongest part of the body (ie. the legs). No spine, back, shoulder, head, etc pain from carrying a shoulder or haversack style bag. Very important for me since I am on foot for long hours during tours. Frees up the shoulders to carry a haversack which carries other non camera stuff as well.
The only downside is carrying capacity and long lenses.