It's way too soon to really pick a favorite. As things are right now, Sheryl isn't the spoiled songstress that seemed to be in ep01. She's very human to say the least, only when talking (and being) with Alto. Both complement each other in a way, and that's good.
Ranka has shown real interest in singing, but contrary to Minmei (thankfully), she's making a steady progress with Alto (know each other - friends - ???) without being too obsesive.
The main problem for Ranka up to now is that Sheryl and Alto have stepped in the "friends" tier way faster, and the interest of the Galaxy Fairy over the Valkyrie pilot is growing stronger. Unless Ranka can pull a full 180º, Sheryl will have the upper hand.