If the measurement is battle power, its definitely Usopp. Both Chopper and Nami have him beat.
Oda's compensation is him recusing the toy-people in Dress Rosa, or saving the Sunny. In Enies Lobby, Usopp would at least take out fodders to help whittle down the numbers, which made him feel useful and like an actual marksman or sniper.
Other than that, he hits people out of his league who are unfazed by his attack, or else successfully wins a 1 vs 1 with a single opponent who happens to be one of the weak members of the enemy crew.