Now this is a lot trickier. Thankfully, I'm biased.
May be surprising that my number one's a difficult choice. But I love love love Airi in Buono! so much more than in C-ute. Strange surprising how much more she's pushed in C-ute - Shock was basically performed by her, with 4 H!P Kids as backup dancers. But I love her interactions with NatsuMomo, partly because I like them both, partly because I couldn't give a rat's arse to pay attention to C-ute out of performances.
Rankings: Momo = Airi + ε > Miyabi, where ε is an arbitrarily small positive quantity
Currently listening to Partenza as I write this (Guess what came in the mail yesterday?). Will do a GETS post soon.
As a bonus, here's something from Rock'n Buono 4.