Luffy has to be stronger than Zoro because, well, he's the main character and wants to become the Pirate King, so naturally he has to be the strongest, even against swordsmen, to maintain that rank in the end. Zoro is the very first crewmate, so I would think that Oda would give more attention to his goal and character development than the development of other characters. Zoro and Luffy share a goal of obtaining the strongest positions for their respective goals; something that the other crew members don't share.
It's too early to see what Brook can actually do, but we do know that he is at least weaker than Zoro. Since there are so many characters in the crew now, I don't think we'll have 1V1 fights anymore for characters other than Luffy, Zoro, or Sanji, because it would just take too long. I would say that most of the emphasis on fights will be on Luffy and Zoro, less on Sanji, and even less on the other crew members.
If Robin was a male, then she would definitely be one of the strongest, but Robin lacks that kind of mentality and is more like the pacifistic knowledge-bearer of the crew, so I would place her in the same category as Chopper.
Franky would come in as the fourth strongest, and Nami and Usopp both use their smarts during battle because they're physically weak and people that we can most often relate to even if we don't want to admit it. I bet you a giant giraffe and a wolf, or a lady that can protrude sharp spikes from her body would make you shit bricks, too.
So my list goes:
Luffy > Zoro
Sanji
Franky
Usopp > Nami
Robin = Chopper
And Brook can either be above Franky or below, but it's too early to tell.