Block difficulty is constantly changing so the system approaches on 10 min block times.
No, it's not. Read & learn, don't just assume.
Read about the difficulty adjustment period. The difficulty is adjusted every 2016 blocks, i.e. about every 2 weeks. That hardly can be seen as "continuously".
"Best" can be quantified by basic quantifiable metrics like transaction speed, value spent, number of transactions, who spent transactions, where in the network they are spent from etc. just depends what the network is designed/optimized for
What does transaction speed means in your idea? Because the "speed" is related to the fee and the miners. So that's not a valid measure. Fee, maybe, but not speed.
Who spent the transaction? You should not know nor care who spent the transaction. Period.
Where are they? Are you kidding me? On a proper decentralized network this should never matter.
Number of transactions and so on are highly dependent on mempool, and, each node has a different mempool, so we cannot compare apples with oranges.
So.. all this "best" logic has shown (I will agree with franky1 here) that you don't know nor care about what bitcoin is and how it works, you just came with some half-baked ideas that cannot work.