hmm… I wonder about that. This probably should go in another thread, but perhaps Brook's lack of real, physical muscles prevents him from getting physically stronger; as if his devilfruit powers magically keeps his body in stasis (unless he drowns to death).
I rank by (one vs one) fighting ability:
Luffy - Zorro (Equal if not very close)
Sanji
Franky - Robin - Brook (Hard to say, but Robin is probably highest in battle potential)
Usopp - Chopper - Nami
Though I'd say it's too hard to rank Chopper, Usopp, Franky, and Robin since we haven't really seen them in 1on1 fights since CP9.
With Chopper - We haven't really seen him fight that much since CP9. He participated so little in the Oz fight to really see if he got any stronger. Regardless, I'd say Chopper's battle potential is the lowest of all the crew. Despite his supposed genius in analyzing weaknesses, he's just a heavy-hitting goon that can't last as long as all the heavy-hitters out there. His fights are always the most boring IMO, because his style never changes - it's always 'hit-&-run' and then 'one-hit-KO'. If he is to get stronger, he's gonna need a new Rumble-Ball, and LOTS more fighting creativity from Oda (more interesting moves/tactics/abilities).
With Usopp - I really think he's grown a lot since the W7 arc, maybe even the most. This arc is the first time he's NOT complained or bitched at all. Hell even during the Oz fight he really participated A LOT (more so than chopper). We saw him jump off a dock in W7 with NO PROBLEM AND take a hit from Oz and get up NO PROBLEM. Shit he's no longer a weak human; he is the closest to his dream :happy:!!!! Can't wait to see him against a serious opponent!
Franky is sloooowwww, but smart and strong. His craftiness compensates greatly, but I doubt he'd overtake Sanji.
Robin is a mystery... always have been. Aside from Yama, we haven't really seen her in a death match or duel. Her ability gives her limitless potential to what she can do in battle. Though, once again, I wouldn't put her over Sanji (yet) cause of the difference in physical prowess.
Oh I forgot Nami.... Yeah physically, she's the weakess. I actually thought her fight with Califa to be utterly ridiculous. Call me a Nami-hater if you want (I swear I'm not), but that weather stick is borderline ludicrous. It's powers are just too much deus ex machina even for One Piece. No need to argue about this... it's just my opinion