Yet arguably it's the most important. The emergence of eukaryotic cells was really what made the diversity of multicellular life we see today possible. After abiogenesis itself, it was probably the most important milestone in the evolution of life on Earth. Given all that, it makes sense to include the distinction between prokaryotic and eukaryotic lifeforms in standard taxonomy.
I can see scope for a whole series here. There's a lot of overlap between cats and dogs, so maybe a joint one for them? And you could do non-mammalian ones too, to mix it up a bit.