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 in survival 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'.