If you consider using glitches, the recently obtained Bedrock in Java Edition is the most difficult to obtain actually obtained block.
A viable theory about how to obtain spawners, End portal frames and such has been developed but nobody has been successful in constructing the necessary contraption so far.
Netherite block, while requiring quite a bit of effort, isn't actually difficult at all - after you've descended to y=8 in the Nether, basic caution against lava and patience with digging will get you the required 36 Ancient Debris, and transforming them into Netherite at that point is trivial.
Comparing the amount of time and effort in a new world, I'd say Mycelium is more rare and sponges are more dangerous and difficult to obtain.
Even stupid Conduit in 1.16 after the fishing farm nerf got sufficiently difficult
it tops Netherite in difficulty.
Dragon head, if counting effort and difficulty 'from start to finish' would be the top block, but it usually 'happens in the process' - it isn't a goal by itself, it just appears near other, more desirable endgame goals.
Another hard-to-obtain block is a creeper head - killing a creeper through explosion of a charged creeper requires plenty of effort if you don't already have a channeling trident.
The Beacon is probably most challenging though.
First, you must kill an obscene amount of wither skeletons - on average 54 if you have a Looting 3 sword, or 120 without.
This done in a Nether Fortress which is rather challenging.
It can be automated but building a farm by itself is quite a bit of a challenge.
Then you must construct a Wither and kill it.
It's possible to 'cheese'
this fight
but it's probably more difficult than the Ender Dragon fight if done 'legit'.
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Risk-wise, the single most difficult block to obtain is the activated Regeneration Beacon, since there are many steps to it, and many of these steps are very risky to make.
Compared to Netherite, it may not take as long to obtain, but it's definitely harder to do so.
Here's why: You have to get into the Nether, which requires getting diamond tools to build a portal.
You have to find a fortress and get to it safely.
You have to obtain 3 wither skulls, which involves killing Wither Skeletons that have a 2.5% chance of dropping a Skull, so you will have to kill 120 Wither Skeletons on average.
You can speed up your progress by getting Looting on your weapon, but that also takes a lot of time.
Next you have to build and fight the Wither, which is the single most difficult mob to defeat to get a nether star.
Finally, craft the Beacon and build 164 mineral blocks underneath it and activate it with a single mineral in addition to have Regeneration activated on the Beacon.
Keep in mind that this is still one single block, even though it requires other blocks (the mineral pyramid beneath) to be in this state.
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I would say Netherite.
Netherite requires four blocks of smelted Ancient Debris (Netherite Scrap) and four blocks of smelted Gold Ore (Gold Ingots) to make one Netherite Ingot.
This means you have to do this nine times to get a Netherite block.
It is so hard to get that Mr Beast made a video about it.
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If one netherite block is hard to get, then a shulker box full of netherite blocks is even harder, and a chest full of shulker boxes full of netherite blocks is harder still.
Using this method, 46656
netherite blocks can be stored in one block, equivalent to 419904 netherite ingots, costing 1679616 gold and 1679616 ancient debris.
Plus the shulker boxes.
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I would say a netherite block,because to get it, you need 9 netherite, which netherite can be obtained by getting 4 netherite scraps and gold ingots.
To get a netherite scrap, you need to smelt ancient debris which is a rare ore only found in the nether, and for gold ingots, you can smelt gold ore.
That should be the hardest block to obtain, unless you count the max netherite beacon.
You can use netherite for beacons, but it is very very very expensive.
For a max netherite beacon, you need 164 netherite blocks(In a pyramid form under the beacon) , and a single piece of netherite to give you the effect of the beacon.
but also you need to get the beacon, which needs a nether star, some glass and some obsidian.
And to get the nether star, you need to spawn and kill a boss, the wither.
I hope this answered your question.
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I think that the hardest to get in the game is the dragon egg.
The only one of it's kind that only has one of it, as well as it being "unmineable" (cause you have to drop it on a slab).
Its also guarded by the ender dragon, so that poses another issue.
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Except for command blocks, structure voids and all other unobtainable blocks, which block is the hardest block to obtain?  For example: an emerald block is hard to get, as emerald ores are rare.
What is the most difficult block to obtain in Minecraft?
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 The recently obtained Bedrock in Java Edition is the most difficult to obtain actually obtained block. End portal frames and such has been developed but nobody has been successful in constructing the necessary contraption so far.   While there are a lot of difficult to obtain blocks like spawners, regeneration beacons, mycelium  and bed=rockk, most people agree that netherite is the hardest to get.
bedrock is suggested Spawners are also difficult to obtain An activated regeneration beacon Most users agree that Netherite is the hardest to get. Mycelium is also suggested.