Rails. In w10, we were one of two non-rails users, though it seems that has changed to larger diversity.
Most YC companies use CircleCI, so we get to see some of the diversity. While I haven't got concrete stats on this, I think it leans slightly more heavily rails than usual (and usual is about 50%). Bear in mind that that's skewed in some ways: if they were using C# for example they couldn't use us.
Via a complete and total misreading of his answer, actually! The initial "Rails." stuck in my mind even after he directly noted that they weren't using it.
Most YC companies use CircleCI, so we get to see some of the diversity. While I haven't got concrete stats on this, I think it leans slightly more heavily rails than usual (and usual is about 50%). Bear in mind that that's skewed in some ways: if they were using C# for example they couldn't use us.