This is a difficult question since it's very subjective basically. Some people prefer sticking to old, reliable sites while some others shifted to newer sites for different reasons as always which has made the market more competitive.
Bitcasino, Sportsbet, Crypto.Games, Stake and Fairlay are some of my favourites that launched before 2021 while Rollbit(Love their trading games) is my favourite site that launched in the first half of 2021.