As you mentioned Johnny Cash at Folsom Prison, it is hands down the best live album I have ever listened to. Why? Because the recording captures Cash, the audience, and everyone else there in such a way that you can feel what a truly extraordinary set of circumstances that took place that day and Cash's personality is right there in the recording. Highly, highly recommended, even if you are not into his music in general it is a historical document and I wish I had come across even a single live recording that comes even close.

If you want some sort of "definitive" edition, I hold the Legacy Edition [1] in very high regard as it to the best of my knowledge contains the two concerts in their entirety.

[1]: https://en.wikipedia.org/wiki/At_Folsom_Prison#Legacy_Editio...