Defences are one of the items that can turn the waves of a battle. The stronger the defence is, the difficulty is to penetrate, and more it is as big as attacks.
Bisu Bisu no Mi is a Paramecia class of Devil Fruit, which grants its user form and control biscuits. This DF was eaten by Cracker and was originally introduced in WCI.
Cracker has excellent command over his DF and with this, he can also create infinity clones built up of biscuits which are nearly impossible to tear. He uses these clone to protect himself from his opponents, and he even uses his Haki in these Clones. Cracker applied these defences to finish Luffy and would’ve worked if it weren’t for Nami.
Doku Doku no Mi is a Paramecia class Devil Fruit, which forms and further controls poison at command. This Devil Fruit’s owner is Magellan, and it was first presented at Impel Down. The poison of this DF is very deadly and can annihilate anyone. The most dominant ability is named the “Hell’s Judgment” which can annihilate anyone in moments and include the smallest touch of this technique can be notably deadly. Magellan can further coat his whole body with this poison, which causes it impossible to reach him without moving through the poison.
Gear Fourth is a technique that is developed by Luffy. Gear 4 Bounce Man, in which Luffy overpowered Doflamingo easily. Bounce Man’s defence is remarkably strong and still, Doflamingo couldn’t beat Luffy. But, Cracker nearly cut off Luffy’s arms in this form.
In this form, his barrier is impenetrable. Not yet cracker could penetrate his defence, and he was repulsed to a faraway place.
It is a Paramecia class Devil Fruit, which enables its user to build and control barriers. This Devil Fruit was consumed by Bartolomeo and was originally introduced in Dressrosa arc.
These barriers are remarkably strong and are difficult to break. They can even hold “The King Punch”, which can believably get down a Yonko. They can also resist the strongest of slashes as viewed against Hakuba.
Jozu is the past 3rd Division Commander of the Whitebeard Pirates, and he holds a Paramecia class of Devil Fruit. This Devil Fruit enables him to turn his whole body into diamond.
Jozu was able to stand Mihawk’s powerful slash. If he could prevent something similar to that then he can stand anything, and it is presumably the greatest defence in One Piece.