I just figured out Roger is weaker than Brook and Chopper. Roger ran from Big Mom. Big Mom had difficulties with Brook and Chopper. If Roger can't face Big Mom but Chopper and Brook can, Chopper and Brook are stronger than Roger.
Usopp got his lap sat on by Robin and had his face consensually buried in Nami's breasts (and she's an ambiguous lesbian). He doesn't deserve to be the weakest. He's the son of Yasopp for crying out loud.