This question has come up many times (here's one thread: https://news.ycombinator.com/item?id=30807759)

Like others have said, pick the tech you want to use first, then find a good one that uses that tech! I would look for reviews, a history of updates, and a founder who knows their stuff.

There's many directories listing them - probably the most comprehensive one is here: https://github.com/smirnov-am/awesome-saas-boilerplates

Good luck!