81/70, the septimal semiaugmented second or septimal ultramajor second is a 7-limit interseptimal ratio of about 253 cents. It is sharp of a subminor third 7/6 by a sensamagic comma 245/243, and sharp of a major second 16/9 by a septimal quartertone 36/35.
The interval is so perfectly approximated by 19-edo, with an error of 0.05 cents. There are a number of edos that do this equally well, 171-edo to name one. The first edo that does this better than 19-edo with patent val is 660-edo.