Believe it or not I'm biased to say Male #2, simply because its what I got over 5 years ago when I started my New Leaf town. It's definitely not the nicest to look at (just look at those all-seeing eyes), but it's grown on me a lot since then. As you could probably assume I initially disliked it even have pilot shades on still to this day but as I discovered with the release of Pocket Camp, my mayor just isn't the same without that face ahaha.
I think my fav face changes all the time, but currently it's #9! Which is funny, because as a kid, I hateeed it. I thought 9 and 12 were super ugly. But they've grown on me as I've gotten older! Ironically, my favorite as a kid was always 5, but now I think it's pretty meh. It's not my least favorite or anything, but it's def not in my top 5 lol. Idk, I just like the more silly-looking faces now!
#1, both male and female. It's a simple look, but they fit better to the overall look of my characters.
I also like #10, because they look like the Lenny face, which is kinda funny.
I feel as if everybody and their mother uses #1, including me on one of my towns, but my absolute favorite is 9. I also really like 12, as my PC manager uses it and I can't see her without it. I actually really dislike #4.
Hmmmm that's very difficult to choose indeed. My NL face is male #2, and though I too didn't like it at first, I really did grow to like it in the end! It looks great with the thick glasses (but that also means I'm not used to my mayor's face without them, lol). In HHD I use male #5 because honestly it looks dang cute and works with almost any kind of clothing & hair colors. There's also something about hte lazy faces of # 7 that I really like. I gotta do something with that sometime