Inspired after playing Pirate Warriors 3...spoilers, of course. I imagine most people are aware of who the current crew is though since it's been like 6 years since the last member joined.
My vote goes to Sanji, personally.
Now, when I first got into One Piece, I thought Sanji rocked---and I mean, on the outside, he definitely has an easy-to-see appeal that many other One Piece characters (especially a lot of the weirder designs) lack for those entering the series. He's got a cool design, and even his timeskip design isn't too bad compared to the others which I think most would agree got overall worse, cool and suave voice (well, except in the 4kids dub...), and he smokes which makes him a badass. Yeah, cigarettes are bad for you, I know that, I don't smoke because of health reasons, but I still think cigarettes look cool and people good-looking people smoking only makes them better looking in my opinion from a superficial standpoint.
But beyond that...Sanji sort of sucks. First off, I feel his development is really wonky. When he was introduced, despite being a relatively normal dude, he was super-strong, and while that didn't bother me at the beginning of the manga when things were relatively more "grounded", as the universe expanded and more and more crazy stuff was introduced, Sanji remaining part of the "Monster Trio" felt more and more off to me---it's just weird he's so strong, and there's really not a lot of good growth when it comes to his fighting skills like there was for Nami and Usopp. I just don't get any sense of progress from his apart from simply learning new moves as the plot demands.
I also think there's not a lot of character development for him either, his goal "find the All Blue!" sort of being one note and his role as a cook doesn't have as much plot-related elements as the other characters. He sort of feels like an artifact almost, a character added early on but doesn't really seem to serve much of a role anymore besides simply being another guy to fight the Boss's minions. The last big role I recall him playing was his undercover stuff in Alabasta and the great joke with Duval, since then he's basically just gone up against various opponents and swooned over women.
And of course, and this is a problem I know a lot of people have with him (especially due to his actions fighting Kalifa), his womanizing gimmick got progressively more annoying and bizarre as the plot went on, when looking back he was actually fairly tame at the beginning. I've heard he apparently has gotten better in Dressrosa, but I think right before, during, and after the time skip he was probably at his worse, and I hope Oda more or less removes this part of his personality and reigns him back in again---it's getting obnoxious, and as I get older, it's gotten more creepy than funny at this point.
Runner up is probably Nami, especially her in my opinion gross-looking timeskip design which I think is embarrassing, especially in the anime when she has the visible crotch lines. She's probably one of my least favorite anime tropes...the "straight girl" who the writers overcompensates for by making her act extremely rude towards everyone else without any real ramifications for her actions and you wonder if she even likes the people she's with, although thankfully that's sort of gone down recently. I'm sure you've encountered this character before---Natsumi in Keroro Gunsou, Otae in Gintama, etc. Let the women have some fun without always making them above everyone else, manga writers.
I also sort of have an issue with Brook too---his character is great and clever, but I feel that they ramped up the Impel Down and timeskip stuff so fast after introducing him I didn't feel he really got enough time with the crew beforehand, and even now after a long time in the real world, he still feels like he's the oddman out. I wish they had one more arc before the time skip with the Strawhats all together to help establish him more.