For me it has to be B4.
The stage as a whole is brilliant, the songs and the order they're in. The outfits, the choreography everything just fits together so perfectly for me.
I really like the songs, my least favourite being kataomoi. This could possibly be my favourite stage.
K5 isn't far behind but I prefered K4 in many ways really.
A5 is a joke compared to A4, really is.