I use this in my Fundamentals classes because it’s big enough that slow queries will actually be kinda slow. Expands to a ~50GB database called StackOverflow2013 with data from 2008 to 2013 data.
It’s easy to learn, has just a few easy-to-understand tables, and has real-world data distributions for numbers, dates, and strings.
I use a Microsoft SQL Server version of the public Stack Overflow data export for my blog posts and training classes because it’s way more interesting than a lot of sample data sets out there.