Gotta go with Brook.
He hasn't changed at all though... Can't help but think his age actually plays a part here.
He is actually very old, he was 88 when introduced and now he's 90 years old.
And what does that mean exactly? Well.. the original strawhats were all young people, they were all teenagers up until Robin whom was the oldest for a while.
So they had this whole thing of going through, figuring out who they are, what they want, discovering their own moral values and who they would like to be as people, typical growing up youth stuff.
Brook however.. he's old, he allready knows who he is! he's been who he is for fifty + years. There's no reason at all to dwell further on it at all, he is exsperienced enough to be relaxed in his own attitude and jokes around on purpose.
At most we have to assume that his fifty years floating around all alone in the floriang triangle left him just... a tiny bit crazy in the nudget.. or a lot.
But he allready knows that and is fine with it. He has no reason to change so well.. and I as a reader is fine with that, I like Brook, I like him a lot.
And I like having him around as a constant support to the more insecure crew members..
Usopp least developed? ... I have to highly disagree with that, he is the MOST developed!
That entire thing from him getting into a fight with Luffy in Water Seven until his big shout on Boing island was one big long character development in Usopp realising he is not just kept around because they pity him, but because there is a genuine need for him.
First when he shouts on Boing Island. "I NOW REALISE! LUFFY NEEDS ME TO! SO I NEED TO BECOME STRONG!" is the character development started in Water seven, or even earlier than that when Usopp asks Nami. "Am I a coward?" coming to an end, a long natural character development.
And we see the end result at Punk Hazard when Usopp stands up to those people in the control room, having his speech.
"You're calling my captain a coward to my face? If that was true this would have been easy, I could run like I want to, but because my captain is like he is, and believes in all of us, I need to do my best to life up to him and his exspectations!"
Something like that, but yeah..
The reason why Usopp was trying to flee as Dressrossa was because well.. total strangers wanted him to fight a fight he knew he could not win where he had no personal gain and none from his own crew had anything to gain from this at all.. Luffy didn't even know where the heck Usopp was and had no interest in this.