It's called PoW. Go for the highest total work in the chain. If you go for anything else you are either allowing your self to be attacked by a malicious branch, or you are yourself attacking the network. There's at any given moment exactly one chain that has the most PoW (by definition). That's the reason why Ethereum is switching to PoS, because they know they can't just take over Bitcoin on total PoW. Be advised that PoS means it's not really a public ledger, but a private chain set up by a very large private club.