You are going to get a bias response here, for sure.

For me, it was Steem Dollars up until very recently but the haircut has killed that off - it is no longer stable. I would also have had bitUSD up there but the black swan threatening there is a turnoff too.

Tether is too risky from a counter-party perspective. I'd probably rather be holding USDC or TrueUSD at this point and I know next to fuck all about both of them.