I feel that Nami, Chopper, and Ussop are underused than Sanji. Nami's won a total of 3 real fights (and lost 1), but she lacks weaponry, and has a more functional position on the ship as a navigator; Chopper's finished 5, but he's a doctor, Ussop's won 4 (and lost 2) but he's supposed to be a shooter, so it's a martial position. Sanji has fought 10 challenges and won, and lost 3. Ussop still needs a real battle in Enies Lobby. He seems best used for fighting Spandam, since they both are muscularly weak, cowards, and uses strange weapons. Spandam seems to be a tactical and strategic person as well.
Although I have to agree that Sanji hasn't fought anywhere near as many fights as Zoro and Luffy. Those 2 pretty much fight magnets. I don't want to tally up their fights; it's not worth the effort.
Oh, and Robin needs to fight more too, but she's still new compared to the rest of the crew.