Abigail is my favorite, because she was my first, and because she resonates with me in terms of interests and personality. Into gaming, into music, with an adventurous spirit, and she likes all things mining. Plus she loves spicy eel! She's just great.
However you asked two very different questions. In the OP, you asked who the favorite is. The thread title asks who is the Best. Objectively though I think Emily is the best female spouse. Here's why:
Reactions to bad gifts: Only Penny is nicer. Emily is clear she doesn't like them, apologizes for it, and doesn't insult you or call you stupid for giving it to her. Abigail and Haley outright call you dumb for having gifted them the undesirable gift, Maru and Leah are nicer, but Emily and Penny are nicest.
Ease of gift giving: She hangs out in the Saloon literally every day other than two during the year where she's readily accessible. She doesn't hide within a locked house like Leah, or wander all over the map like Haley or Abigail, she isn't way out of your way like Maru, or have a highly variable schedule like Penny. She's almost always in the Saloon after 4, and they sell things right there which she likes if you forgot to bring one of her Many loved gifts.
10+ heart events: That which is locked behind pursuing a relationship with her. Leah literally punches someone during her 10 heart scene, and expresses no real interest in you, instead just yelling at someone else. With Emily, a potential point of conflict goes, *ahem*, a very different direction. At the 14 heart event, Abigail gets traumatized. Emily gives you a unique gift.
There's lots of reasons to prefer one over another, but Emily has in game mechanical advantages over the others. So I think it's fair to call her "best".