My favorite is...actually I like them all.
But if I had to choose then Mezame, The Song of Storm and Fire, Dream Scape, Houseki, Zodiacal Sign. See I can't just choose just one. But yeah, I guess my only gripe is the short abrupt background chorus in Song of Storm and Fire but I still like it. By the way I remembered, I didn't like Canta per me, the opening part sung by Yuriko. She used a bit too much of her voice-shaky skill (what do you call that?).
As for which one dropping out ruining the group, then I agree it's definitely Keiko. But if any one of them drop out it would still affect the whole group! They're so good as a group.