Mia takes the top spot for me because I'm partial to the childhood friend/hopeless romantic type of characters and with romance being my favorite part of these games, her route should be the most appealing one to me as I expect it to be the most wholesome one.
Sophie is just a tiny bit behind Mia, with Bree and Eve tied at number three. Like Subnero said, they are all likeable so putting one above the others doesn't mean there's anything wrong with them. In fact, they all fiiine.
Of the side characters, I'm not really interested in the MILFs but I'm looking forward to seeing Jenni again. She's hot and there's just something about her... maybe how she handles that lollipop.