What is the best Minecraft block in terms of undistructable blocks?

For the definition of "indestructible block", the material in question MUST be solid (which eliminates Air, being a gas or the default "void" block as well as liquids like Water & Lava), must exist within the core editions of Minecraft (being Java & Bedrock editions, which eliminates all modified blocks added into the game) & it cannot be destroyed within the normal bounds of the game under survival mode.

By this definition, Bedrock would be considered indestructible for all practical purposes.  Granted a Bedrock block has a blast resistance rating of 18,000,000 & cannot be mined nor destroyed within survival mode (this can ONLY be done in creative).  Obsidian, for comparison purposes, only has a blast resistance rating of only 6,000 (or 3,000x less resistant compared to Bedrock).

While there will be a few more blocks that will have comparable blast resistance coming in version 1.16, Obsidian is currently the only near-indestructible block in the game that you could utilize in a survival build.  Please be aware that Obsidian is a pain to harvest, as it takes around a minute of constant mining with a diamond pickaxe to pick it up.  Without the diamond pickaxe, it takes considerably longer (5-20 minutes, depending on the pickaxe utilized) just to destroy the block.

Hope this helps!

Has to be Obsidian

nothing else really comes to mind

water. its immune to explosions, endermen, fire, and it acts like a shield if properly placed over a house that you could walk through, and its super cheap. 

but if you're talking about generally, try obsidian 

Obsidian if you're looking for a block that's obtainable in survival/adventure.

Bedrock is unbreakable but you can obtain it without creative mode