This is a hard question! I actually really enjoy watching each of the female characters on this show... they all add something distinct to the show. I really like Emma though. I think it's very very hard to capture "ordinary girl" lead without becoming boring, but she just makes it believable, her personality really reflects what I'd expect someone with a hard life like hers to have, and she is very likable. I feel like I can relate to her, and she does a great job leading the show. Runner up for me would be Belle and then Regina.
Regina hands down. I haven't started watching this season just yet but not Snow White. She's way too idealistic, happy go lucky. Belle is alright, a bit too soft for my taste. Emma is a close second. Pretty reasonable most of the time, a bit of a bad ass but I didn't like how hard they tried to push her romance with Hook.
Regina is 'bad' but always has reason behind what she's doing. She's not fucking around for shits and giggles.