which is the best application for blockchain?
Bitcoin is the best and only sensible application for Blockchain:
Bitcoin needs its blockchain to function and there would not be a functioning blockchain without a native currency (bitcoin) to properly incentivize resources to protect it. That native currency must be viable as a form of money because it is exclusively what pays for security, and it must have credible monetary properties in order to be viable.
Without the money, there is no security and without the security, the value of the currency and the integrity of the chain both break down. It is for this reason that a blockchain is only useful within the application of money, and money does not magically grow on trees.
This is also the reason why e.g. commercial 'blockchain solutions' with proprietary blockchains make no sense. Because you need decentralized mining which is best incentivized by receiving money for your energy expenditures. So, the blockchain needs Bitcoin. The blockchain was made
for Bitcoin and sure, you can try to build other things on it, but people will prefer Bitcoin over a currency that tries to do 10 other things on the side.