There are a number of islands the Straw Hats have visited in the world of One Piece, each with its own unique cultures and histories. Some of these islands are beautiful oases where the Straw Hat Pirates are able to enjoy themselves to their hearts' content. Others are...not.
There are multiple factors that make some islands far worse than others, such as the ruler, the wildlife or even the climate. However, to be called one of the very worst, the island usually has problems in all these categories, and maybe even more that are completely unique to it.
Starting off the list is the Germa Kingdom. As one of the most technologically advanced nations in the world, if not the most, the Germa Kingdom might seem like a good place to live. However, upon closer examination, the darkness that permeates throughout the entire nation becomes apparent. Driven solely by the ambitions of its ruler, Vinsmoke Judge, the Germa Kingdom and all its citizens exist only to serve him.
From what's shown in the manga and anime, Germa has practically no citizenry aside from staff that serves (and is frequently abused by) the Vinsmoke royal family. The rest of the nation is inhabited by genetically enhanced and modified clone soldiers, which just so happens to be Germa's claim to fame. Embodying everything cruel about science, Judge is willing to do whatever it takes to achieve his goal of conquering the North Blue, including experimenting on his own children. If Sanji hadn't escaped the nation as a child, he would have surely died, regardless of his status as one of the nation's princes.
An island with a dark and sordid history, Punk Hazard is easily one of the worst places to live in all of One Piece. With half of the island being perpetually on fire while the other half is eternally frozen, there is very little comfort to be found here. Ruled by Caesar Clown, a twisted mad scientist if there ever was one, all who live on the island are subject to unspeakable cruelty.
Paralyzed from the waist down by a nerve gas created by Caesar, most of the inhabitants have had to have their bottom halves replaced with those of animals and serve him. His new subjects don't even realize the nerve gas was created by him, instead viewing Caesar as the man who saved their lives and gave them the ability to walk again. The same goes for the children on the island, all of whom were kidnapped and experimented on. While the Straw Hats did manage to save everyone and depose Caesar, the terrible weather conditions and landscape still make the island completely inhospitable.
One of the strangest "islands" on the seas of the Grand Line, Thriller Bark is actually the ship of the feared pirate and ex-Warlord of the Sea Gecko Moria. Thriller Bark started out as an island in the West Blue, but through unknown means was transformed into Moria's very own pirate vessel. Sailing the waters of the Grand Line's Florian Triangle (think the Bermuda Triangle but spookier), what makes this island-turned-ship so interestingly inhospitable is mainly its own denizens.
Aside from the evil yet ultimately underwhelming Gecko Moria, the island is home to his depraved crew, consisting of multiple sex predators and young women with the power to make you hate yourself. If that wasn't bad enough, one of the aforementioned predators is a mad scientist who, with the aid of his captain's Devil Fruit abilities, created an army of zombies and zombified creatures. These zombies hunt down unsuspecting visitors in the hopes of adding their ranks to their already frightening army, making this island one of the last places travelers would want to reach.
While a beautiful island with captivating architecture and landscapes, throughout Donquixote Doflamingo's reign, the island was a veritable nightmare. Known as the country of love, passion and toys, Doflamingo was able to create a perfect facade for all that visited the island. The harsh reality was that all the living toys on the island were actually once people who were turned into toys by Doflamingo's subordinate, Sugar. Not only were all memories of the toys' previous lives erased from everyone they knew, but they were also turned into slaves who had to work in secret factories during the night.
If all that wasn't bad enough, there was also the constant threat of Doflamingo himself to worry about. The Heavenly Demon was responsible for multiple tragedies throughout his reign, including when he used his Devil Fruit's powers to turn the nation's army into his puppets. He then used the army to slaughter the citizens, blaming the then-king for their actions and forcing him to abdicate the throne. His "birdcage" ability also managed to trap all the citizens inside a cage made of super-sharp string which, over time, would shrink until it had cut through and killed every last citizen on the entire island. The island would have succumbed to that fate if it weren't for a certain rubber boy.
The island the Straw Hats are currently visiting, Wano is one of the show's most incredible islands, and if it weren't for a specific Emperor of the Sea, would be an ideal place to live. Kaido, the aforementioned emperor, has turned the once peaceful and prosperous nation into a shadow of its former self with the aid of the current shogun, Kurozumi Orochi. Samurai used to be this country's greatest protectors, but they have been either been killed or imprisoned and forced to work in weapons factories. These same factories have also polluted the once bountiful land of Wano so that both its water and food is poisonous to most.
Due to the lack of natural resources, many villages have become poor to the point of destitution. Orochi, in the cruelest and most twisted sense of charity, decided to feed some of these villages failed SMILE fruits. While there is normally a small chance of gaining some form of animal ability with these fruits, the ones he gave these villages were already known to be duds. This meant that anyone who ate the fruits is now forced to live the rest of their lives with a perpetual smile and eternal laughter, regardless of how much pain or sadness they are feeling. The atrocities Wano is experiencing have spurred the Straw Hats to liberate Wano at any cost, but until they have accomplished this goal, the island should be avoided no matter what.