Day 6: Your favorite subunit?
This one was pretty easy for me. To me, the one-shot units in H!P (as much as I love many of them, esp. Elegies) didn't really count as true sub-units. They didn't run long enough to establish great discographies, and I usually need more than one song off which to base an opinion.
There isn't a single Tanpopo song I don't love, and each my favorite members from 1st, 2nd, 4th and 5th generations appeared in this group over its 4-year lifespan. Beyond my biases, Tanpopo's songs are proving to stand the test of time, as we see with all of S/mileage's awesome covers and others' performances in concert, and that to me is what makes a truly great subunit.
Runners-up (units I see as having great staying power): Not Yet, Petitmoni, DEF. DIVA, Watarirouka Hashiritai.