2-limit
A 2-limit interval is either an integer whose only prime factor is 2, or the reciprocal of such an integer. Naturally, since 2/1 is the octave, this limits us to unisons, octaves and stacks of octaves. The 2-limit is fundamental to any sort of EDO, and in fact, 1edo is basically the 2-limit in full, only with additional temperament properties for other primes.
Due to octave equivalence the 2-limit is also very valuable as a transparent extension to various prime axes.