Given two chains A & B which are different but equally valid without relying on a trusted third party tell me which one is the "best" chain?
This is solved by verification, code review etc. etc.
Those are vague nonsense answers.
Code review? What CODE will allow you to verify which of two chains is the best if not using criteria that longest chain is best chain.
Verification? What can you verify to determine which chain is the best if not using the criteria that the longest chain is the best chain.
etc?
Give me specifics. You are a brand new node bootstraping today you receive from peers two equally valid yet different chains A & B. The code in the client can be assumed to be without error. Chain A is longer, thus by the Bitcoin consensus rules a Bitcoin node would report that Chain A is the best chain. You seem to believe that nodes could reach a consensus that B the shorter chain is "best". How exactly would a new node reach that conclusion. Without specifics that is simply imagining the problem away. Some magical as of yet code will allow the client to pick Chain B because it is the better chain despite being shorter is not a real answer. What exactly could the new node do to determine that B is the better chain over A?