Depends on where and preptime. In Shuma's realm, Shuma. Dormammu's realm, same thing, except Dormammu. On earth, again, it depends on circumstances. Shuma-Gorath is older and more powerful, but there are special bindings made exclusively against him and to prevent him from coming to earth in the first place. Dormammu won't be so weakened. So on earth, normally, Dormammu. But if Shuma was able to cross over at full power, he'd win.