The major third. In 31-ET, the major third is so close to the natural interval of the harmonic series that this interval can practically be treated as a perfect interval in this tuning: it is a closer match to the natural interval than the fifths and fourths are in 12-ET.
As in 12-ET, the major third can be created by stacking two whole tones. Compared to 12-ET, the major third in 31-ET is quite a bit narrower, and sounds much more in tune, and thus more consonant and stable. In 31-ET, however, stacking four major thirds falls short of an octave by a diesis.
The major third is a strong harmonic interval, and implies the bottom note as the root; the only intervals usually able to overpower this strong harmonic sense are the fifth and fourth.