Is this based on how everyone would do in a serious fight, or how they normally fight? Because if Usopp were to fight without being frightened for even a second, I'd place him as second strongest after Luffy.
His marksmanship skills are nothing short of extraordinary, and he could easily disable someone like Zoro or Sanji from long range. He could fire pachinko balls at Zoro's thumbs, for example, while calculating the angle of Zoro's dodge. Breaking his thumbs would make him unable to wield swords effectively, thus giving Usopp and even larger advantage. (just one example out of many I could give) In a serious fight, I think Sanji would be completely outclassed by Usopp, as well.