Wait… I got lost x_x
what strengths are we comparing? Speed? Physical strength? Brains? Cuz there's so many different things to consider that -imo- it’s not possible to determine how strong each character is in relation to everyone else... I only always assumed that luffy/zoro are a bit stronger than sanji because sanji does way more than they do on the ship (and yet he's still one of the best fighters) and doesn’t need to fight anyway. Also, luffy's the kind that will keep hitting no matter what without thinking very much or treating people differently (lol he even hit vivi). And all zoro usually does is sleep/train/fight, so I expect him to have earned a little from that to excel in something otherwise he's just not that useful.
I don’t know if enemies fought = value of strength since a lot of time the match ups just "happen" =P but I think we can assume that luffy> everything XD he's just so random. Actually. -Better yet- mugiwara>>>>everything cuz whatever enemies they face, they always win and that’s what matters really. Crew VS crew anyone?