Tether USDT is by far most popular stablecoin and it has biggest marketcap and volume from all other stablecoins.
However, I don't consider this to be the best stablecoin since tokens can be frozen at anytime and for any reason.
I would not suggest any stablecoins long term, they should be used for trading only.
Tether is the largest stablecoin provider in the crypto market. Their USDT is very popular and has marketcap of over $100B. I think that's enough for us to trust and use USDT. Tether has also been accused of reserve proof, but no one has been able to charge Tether, and Tether is constantly improving the quality of its reserve proof to strengthen user confidence.
Tether is not a public blockchain, Tether only uses smart contracts to mint and burn USDT. As an ordinary investor, we also do not need to worry too much about the possibility of account freezing. I haven't even thought about it in years of owning USDT.
USDC from Circle may be a pretty good alternative, but it has also been depegged quite seriously, so I still choose USDT for my balance when DCA crypto.