I was looking to create a vault that can also be used in survival multiplayer, but the only problem with that is the power of players being able to just break the blocks, so I wondered what block is the hardest to break, and have the ability to be pushed by a sticky piston? Obsidian and crying obsidian cannot be pushed, but netherite blocks and ancient debris can and they're as hard as obsidian, but extremely expensive, and if a player does break throught them, they would just get free wealth.
(block of netherite = 50; ancient debris = 30; block of coal/iron/diamond/etc = 5. There is also hardened glass, 10, but it only exists in Bedrock Edition)
Also, even obsidian/netherite only takes a couple seconds to mine with an Efficiency V diamond/netherite pickaxe so it isn't much of a deterrent except against players without diamond+ tools (even then a determined player with plenty of time could break in; using the wrong tool will still break blocks faster than by hand and even an unenchanted wooden pickaxe halves the time from 250 to 125 seconds and gold only takes about 21 seconds, less with Efficiency, so if they only had to remove two blocks to get in they could do so in 1-2 minutes or less with basic tools).