This is really difficult to know, most likely the most popular answer is going to be the binance DEX but it is fair to question us if it is really a decentralized exchange when binance is the institution that is behind it, the truth is that most decentralized exchanges are lacking still many features that you can find in almost all centralized exchanges and that is why they are not so popular, so it is entirely possible that the most popular DEX during the next two years has not been created yet and it is still in the development stage.