USDT is the most popular option. First of all, it has the biggest trading volume from all stable coins so it is very liquid. Secondly, it has the biggest number of pairs on different exchanges so you can freely change it without long-long routes. I also accumulate exactly USDT when I quit for the time of bear market so as to enlarge my portfolio on the dip. But if you need to hold a big sum of money, you should consider creating a basket of different stable coins.