The chain Freddie's has really good custard. It's seems the same as soft serve ice cream ... not sure the difference, but it's really good. Different flavors/toppings.
The best was the long-since-discontinued tart frozen yogurt that Sweet Green used to serve...
But my daughter and I have become recent fans of the Nothing Fancy soft-serve (which is available at Pineapples and Pearls, I think until 3 pm or so). They have a butter-popcorn flavor that sounds gross but is actually pretty heavenly.