As far as I know bustadice pays the highest maximum reward on a single bet followed by primedice and Betcoin and lot others but its literally impossible to high the maximum outcome unless you are enough luck to win the lottery.

Anyway here is a list : https://bitcoinplay.net/bitcoin-dice/ Which contains all the information along with maximum reward as well but don't trust the list completely just use it as reference and cross check the information on the official websites before going for it.