Primedice is the most popular and most trusted.
PRC has the largest max profit per bet at ~280 btc at its current bankroll size.
Sharpdice has a very low house edge of ~0.1% for relatively small sized bets, while Safedice has a 0.5% house edge for any bet with a profit <= 8.87 btc at its current bankroll size.
The dice game on Nitorgensports has a jackpot which makes the EV positive from time to time.