Rails itself is tested in Minitest.
I found recently that when I wanted to write a Gem that interacted with parts of Rails, Minitest was easier because I could go off the Rails minitest examples.
Most large production apps I’ve worked on are tested in Rspec, which is by far the popular choice. I do find that over-use of “declarative-style” testing to be difficult to read. (I.e., when so many things are defined with “let” statements that you have to reverse engineer 6 methods just to figure out what calls what). Having said that, some people really like the declarative style of Rspec.
-Jason