A rescue of Mary Frances was of a clump in place for over 7 years, bound by concrete on all sides forcing growth on top of each other. When all dug and separated I found not one sign of rot. Great sized blooms, good show and no rot in subsequent years of blooming. I will be leaving many clumps behind when I move.
Gypsy Queen is another that seems to be very hardy. I have around 7 clumps and no signs of problems. Again the new owners will "benefit" from them. I just hope they like irises.
And there is Lent A Williamson, another very hardy and prolific one.
And Spartan, Saturday Night Live, Dusky Challenger, and more.
A measurable characteristic would be great, but all seem to be somewhat subjective when it comes to hardiness, with lots of variables.