How doesn't the most powerful miner get the block reward everytime?
You need to build a valid block. That means that you need a valid nonce. If you start searching from 0, 1, 2, 3... or wheather you try random numbers is irrelevant.

The hash is also not only dependant from the nonce. If you start at 0 and another miner starts at 0 and you both have a different set of transactions in the block, you need to finde different nonces.