1. Void Structure The Void Structure can only be obtained by commands, and here’s the only way you can break it: Creative, Operator, and Tricky if you aren’t holding it.
2. Deny / Allow / Border Block / Barrier You need to be on Creative and Operator to break it, and you cannot see the Barrier if you aren’t holding it. These blocks are also only obtainable by Commands.
3. Bedrock Bedrock cannot be obtained on survival, and also naturally spawns at the bottom of the world. You can get it from the Creative inventory.
4. Obsidian Obsidian can be created and found. Obsidian is created when you place a lava block into flowing / source water block. Mining it with a Diamond Pickaxe is kind of long. Obsidian can also be found in ravines, when water flows into lava. Obsidian can also be found in the end, where towers of obsidian loom at all heights.
5. Reinforced Sculk Block I do not have a lot of information about the Reinforced Sculk Block, as I have never found one in the wild. It has been added into Minecraft from the1.19.2 Wild Update, so I cannot provide you information with it. If you know anything about it, Comment below! I’ll add your comments into this post myself.
6. Ancient Debris / Netherite Block These blocks cannot be exploded by explosions, (I’m not sure about the Netherite Block) and have the same amount of time mining it.

The list goes all the way down all the blocks in Minecraft, but I just did these. If you have any more, comment down below. Also, here is the button to go to the easiest blocks to break in Minecraft. Have a great day!