H!P takes the cake when it comes to a cappella:
speaking of W and Aya, I've always really enjoyed these two performances showing that just because Nono and Aibon weren't in Musume anymore didn't mean they still didn't cause trouble:
https://www.youtube.com/watch?v=n3XBCU8VtDA
https://www.youtube.com/watch?v=9a3RUT5W8aI
And here is a random performance of Momo because why not.
https://www.youtube.com/watch?v=kAFvsTsg_cU