Right now Rainbow 7 and Sexy 8 Beat are getting the most play time by far. But Second Morning is what got me into Momosu in the first place and I'll always have a special place for it, then there's the fact that Ikimasshoi! is just pure genius.
I'll go with Ikimasshoi as I think that's my long standing favourite, I felt the others deserved an honourable mention though.