If we talk about the safest stable coin then go for DAI because this one is a decentralized stable coin, this is only I know a decentralized coin. I don't think if there is another decentralized stable coins in the market. USDT and BUSD are good stable coins but I don't think if these two are very risky to store your funds in the long run, they were exists for many years and not having any problem especially USDT.