It all depends on your priorities. Most people play blackjack to have fun, in which case the “best” version of the game is whichever one you actually enjoy playing the most. Since online real money blackjack games can be played with very small stakes, it’s well worth having a few hands of each different blackjack variation to see which one you prefer.
If your sole focus is on RTP and you’re playing blackjack to make money, then single deck blackjack is the version that will be most profitable for you in the long term – providing you use and stick to basic strategy rules.
Variations are a lot of fun and keep things interesting. But when you get games where Blackjack is only paid out at even money, it’s bound to have an effect on the house edge long term.