One Piece is one of the longest running animes out there, which means that the characters have also had plenty of screen time. Characters need to grow and develop over time to stay interesting to viewers, and the same is true for the cast of One Piece.
The Straw Hats have all had extensive backstories, as well as several arcs before and after the time skip. They've grown closer to each other and made their names known all across the Grand Line. They've all changed in many ways, and not all of those changes are just physical either. No matter who you look at, they are basically unrecognizable from when they started out.
9 Nami
Nami is the navigator of the Straw Hats and one of the first to join the crew. Her core personality has stayed the same, but a major difference through her years is her confidence. Before the time skip, she would act cowardly and be unable to help her allies in battle. Now she's a force to be reckoned with and has performed several admirable feats for her crew. Physically she's grown her hair from shoulder to waist and has changed to appear less like a girl and more like a woman as she grew up.
8 Zoro
Zoro is the fighter of the Straw Hats and the first crew member Luffy found. Mentally he hasn't changed much through the years. His personality is the same, and his values haven't changed. Physically he is much different than before. He's gained considerable muscle mass due to his extensive training with Dracule Mihawk. At some point during his training, it also appears that he lost an eye, as he now has a scar over his left eye and never opens it. Otherwise, he's the same old Zoro.
7 Usopp
Usopp is the sniper and inventor of the Straw Hats. He's gone through some of the most dramatic changes of the bunch. Early in the show, Usopp was a wimp and a coward who only fought when forced. He was also very self-centered, referring to himself as Vice-Captain several times.
After the time skip it's clear that he has been working out and is now packed with muscle. He's also more confident and mature, although he still has bouts of cowardice. He now accepts his place in the crew and does his best to keep them safe.
6 Sanji
Sanji is the chef of the Straw Hats and is one of the least changed of the main characters. Physically he's packed on a little muscle and grown his beard a bit, but otherwise, he's exactly the same. He's much stronger now, but all the Straw Hats grew stronger after the time skip. The main emotional change he went through was fully accepting the Straw Hats as his family. After dealing with his biological family at Germa 66, he threw himself wholeheartedly into protecting his new family.
5 Chopper
Chopper is the doctor of the Straw Hats. Emotionally he's mostly the same, although it's obvious after the time skip that he's grown in both confidence and strength. Physically all of his forms differ a bit from when they first appeared. His default form got a bit cuter, whereas the others increased in size and effectiveness. His true reindeer form changed the most, growing large enough to carry a couple of people with shaggier fur and larger antlers.
4 Nico Robin
Nico Robin is the Straw Hat's archaeologist. Physically she's only changed a little bit since she was introduced. She now has longer hair, lighter skin, and a more full figure. Emotionally she's changed the most out of the Straw Hats.
She used to be a villain but later joined the crew for protection. She was constantly worried and unhappy since she was wanted by the World Government. She's relaxed tremendously after the time skip and is now genuinely happy to be a part of the crew and would do anything for them.
3 Franky
Franky is the shipwright of the Straw Hats and has undergone the most drastic physical changes of them all. Since he is a cyborg, he is able to upgrade himself as he sees fit. He went from a large human to gargantuan, with massive metallic arms and shoulders. His legs are also now obviously robotic, and he has many new features packed into his big body. Emotionally he's still the same, and it doesn't seem like he's had any big personality changes over the years.
2 Brook
Brook is the musician of the Straw Hats. He's also gone through a big physical change, considering he literally died. He's changed into a full skeleton and ditched his original clothing for a more flamboyant costume for his role as Soul King. Besides his physical changes, he's mostly the same emotionally. The main difference is he tends to be more fierce and daring now, like when he fought against Big Mom knowing that he would lose.
1 Luffy
Luffy is the captain of the Straw Hats and the main character of the series. Physically he's had two distinct changes. He now has a massive scar on his chest from an attack by Akainu. He's also much more muscular thanks to his hard training during the time skip. While many aspects of his personality are the same, he's definitely matured. He also takes things more seriously after the time skip, even if he does still have childish moments. He's slowly growing into someone capable of becoming King of the Pirates.