Most likely I will choose tether as this coin pleases me very much recently. All other altcoins lower its price but tether continues to rise up. Probably at this moment is the clear leader among altcoins. But if you want to invest then don't forget that yato will soon be giants stablecoin - libra and master card
Why do you mention altcoins when it comes to stablecoins? From the name "Stablecoin" it follows that this coin is stable, the situation on the cryptomarket does not affect it (there are no its pumps/dumps), because in most cases its value is tied to Fiat currencies.
Your Libra will never become a real cryptocurrency, it's just a payment system tied to a social network.