Proof-of-Work public blockchain security is measured by the processing power (aka hashrate, because it calculates cryptographic hashes) that the network spends on security. Bad actors need to get a majority of the hash rate to attack the network and even so they are limited to what they can do. Bitcoin is the most secure because it has the highest hash rate, so much so that even global companies and state actors cannot really be incentivized to amass that much processing power.