B-U-O-N-O Let's GO!
Oh wait, done that already.
Not much to choose from. There's Aa!, which is basically Buono! minus. Yume to Genjitsu is a favourite of mine, especially since Airi and Miya trolled Momo in a Buono! concert. One of the extras were other outtakes of that scene, my favourite being Momo going along and the other two having to stop her XD.
Quality-wise though, I must bow down to High King.
A unit made up of some of the best dancers in H!P. I'm not the best judge for dancers, but the quality of their songs have been pretty high. Cinderella Complex was my alarm ringtone for a while, while the rest of their discography are solid hits. Even Disney Love.
And they get good costumes as well. Those purple dresses in the Shuffle Date concert - classy, eh?
Would like to shoutout to my favourite non-H!P unit: Watarirouka Hashiritai 7.
Cutest unit ever! Solid song lineup. Well, unique at least.