Favorite as people or as stories? I guess stories, since favorite characters don't tend to come in groups.
I voted for the Solarians, not because I like Solaria but because her group has more going on - in particular, more going wrong. All three groups have problems, but Solaria's has (it seems to me) more things they care about, and more conflicts and failures, and therefore more suffering. And sympathizing with characters as they suffer is a big part of what makes stories interesting.
I think part of what's going on is that, while Araceli and Fatima's groups have big scary threats like being hunted by Xantrich or Shirande, the threats are so grave that they can't be realized. So they scare the characters, but not the readers, because they know things will turn out OK. When Haz threatened to execute Fatima, he was bluffing - killing her off would cripple the story, so we knew it wasn't going to happen. When he threatens to have Lavian get burned to death by the man she loves, he's not bluffing, so it's much scarier to the reader. If it does happen, Solaria will probably feel guilty about failing to save her, and Milliene will try to comfort her, so it means more story, not less. Similarly, if Milliene turns out to be hopelessly in love with Solaria, or if Aramande reaches Palrm Temple first, the story will be advanced, not hindered.
I wonder how much of this is in my head rather than in the comic, though. These threats are credible, but none of them has happened yet. I may be reacting to the stories I imagine rather than the ones Haz has drawn.