To be fair, most of the significant 1 on 1 fights go to Luffy, Zoro, and Sanji. Usopp and Chopper have a decent amount. Nami, Franky, Brook, and Robin all could use a major battle or two more.
I'd much rather have the fights the way they are in OP. The only reason certain characters haven't each had 1 on 1 fights is because Oda does not fit his villain teams to all be perfect foils of the Straw Hat Pirates. Aside from Enies Lobby, there haven't been any big 1 on 1 fights since Alabasta. Some series like Reborn! do it every single arc. It's better the way fights are done in One Piece….
Let's just look at all the Straw Hat Fights....generally I'd say Luffy, Zoro, and Sanji have the lion's share. Robin and Brook are the lowest. Keep in mind Robin joined later in the series. Skypiea didn't have many major one on one fights. Neither did Thriller Bark. Enies Lobby revolved around Robin and was an excellent story arc / saga in every way. Having her fight a major opponent wouldn't have worked for obvious reasons. Shortly after TB the crew was scattered. Yes, Hammond was a curb stomp...but so were the rest of the NFP. I don't think it's as much sexism as it is a matter of opportunity. Robin hasn't been around for a SHP vs. Major opponent showdown (which Oda uses sparingly; so far only Baroque Works and CP9). She was with the crew for the CP9 fights, but as said, didn't fit in those fights.
Just looking at the major Straw Hat Pirate fights....
Luffy (18) - Buggy, Kuro, Krieg, Arlong, Mr. 3, Wapol, Crocodile, Bellamy, Eneru, Foxy, Aokiji, Blueno, Lucci, Moria, Oars, Boa sisters, Magellan, Hody
Zoro (9) - Cabaji, Butchie and Siam, Mihawk, Hatchan, Daz Bones, Ohm, Kaku, Ryuma, Hyozou
Nami (2) - Miss Double Finger, Kalifa
Usopp (4) - Chu, Mr. 4 & Miss Merry Christmas, Perona, Daruma
Sanji (8) - Pearl, Gin, Kuroobi, Bon Clay, Wanze, Jyabura, Absalom, Wadatsumi
Chopper (6) - Chessmarimo, Miss Merry Christmas & Mr. 4, Gedatsu, Kumadori, Hogback, Dosun
Robin (4) - Pell, Yama, Hogback, Hammond
Franky (3) - Nero, Fukuro, Ikaros
Brook (1) - Zeo
Jimbei (1) - Wadatsumi
Vivi (1) - Miss Father's Day and Mr. 7
Significant team fights that I think should still be mentioned as well:
Luffy, Sanji, Usopp vs. Satori
Straw Hat Pirates vs. Pacafista
Straw Hat Pirates vs. Oars
Robin and Chopper vs. Hogback
Usopp and Chopper vs. Mr. 4 and Miss Merry Christmas
Sanji and Jimbei vs. Wadatsumi
You can clearly see Robin, Franky, Brook, and Jimbei haven't had any real major fights (aside from Franky vs. Kumadori). Nero, Yama, Zeo, Hammond...all curb stomped pretty much.
It's always been clear that Robin is pretty powerful. Oda's also made it clear recently that both Nami and Usopp are a force to be reckoned with. The lack of major fights for Robin at least is a function of the way he's telling the story now more than anything as evidenced by Franky and Brook's lack of major battles. Nami has had 2 major battles. Usopp has had 5...it seems likea big difference, but compared to Zoro and Sanji none of them really have close to the same amoutn. I wouldn't turn Robin's lack of major fights into any form of sexism...because it's not. NFP were not major fights. Nero wasn't a major fight. Nami is the one with the least fights relative to the amount of time she's been in the story...but Nami and Usopp were weak in the first part of the story. Usopp has 1 extra fight for the Arlong Park arc in which he was fighting on Nami's behalf. Another additional fight on Thriller Bark. TB was after Oda made the switch to a few fights per arc... a GOOD change. The matchups were perfect as is so Nami had no room for a 1v1 fight. The arc was already 50 chapters. It couldn't be any longer really.
All that said, I would love it if someone like Erza from Fairy Tail joined the Straw Hats!