I have been experimenting with pretty much all of the rare non-trifoliate hardy citrus trees.
My results so far seem to show that Changsha is the hardiest of them. Yuzu is a close second, but does not seem to be as hardy as Changsha. But I cannot be absolutely sure about this.
All of the research from things I have read have informed me that Ichang papeda should be the hardiest non-trifoliate citrus, but that is not what my experimental observations have shown. I had obtained Ichang papeda from two different sources in the Portland, Oregon area but it is possible they might have originated from the same source and could have perhaps been grown from some specific seedling that had less hardiness than its parent, though I think this is not so likely.
Plants were grown in the Pacific Northwest, climate zone 8a, and last winter had a low that may have gone down to as low as 9 degrees F one night, though other than that it was not a very cold winter. It killed some of the Yuzu plants, even one that had been surviving for several years through cold winters before that, but a Changsha that was planted in a very protected spot and sheltered by a large bush growing around it (although it was not covered) survived and even kept all its leaves.
To be fair, I have not grown Prague Citsuma, but another member of this forum is growing one in a protected spot and in the middle of a suburban neighborhood across the bridge from Portland (also zone 8a). Its leaves did not seem to look as good as the Changsha he was growing, which one might perhaps take as an indicator of cold tolerance. I got to taste the inside of the fruit. It was very much like Satsuma mandarin but the fruits were smaller, more sour, and perhaps just a little bit less ripe tasting, but not bad at all.