Atlus is probably the only game company that makes me love every main character. I think what they were aiming for is to try to make all phantom thieves created equal, and in a way, they did. If you just remove persona preferences, then you end up liking everyone. Sure, some characters are more fleshed out than others, but in the end, you have a happy-hungry bunch of thieves. I guess the only exception is Akechi and Morgana, where you either love them or hate them, and it is determined just by your tastes and values. Personally, I really like Akechi, and Morgana is okay I guess. Let's take a look at each character:
Ren: He's the sexy sassy boi that created the team. He is likable because he's quiet, but he is super bad-ass. If he's your favorite character, then it's probably because of his appearance and sexiness, and personality.
Ryuji: Okay, you can't not love Ryuji. He's a silly but determined bro who always has your back and will never betray you. He's likable because of his funny lines that always make you laugh, the memes, and his bro-ness.
Ann: She's a hot independent girl, and is determined like Ryuji, and will always help someone out. That's why she joined the thieves. She's likable because of her bad acting, but she can also be sweet and compassionate at times. She's also very energetic and lively. It may not be obvious because of her looks, but she's a total tomboy.
Yusuke: He is my favorite character because of personal stuff, but if we take that out, he's still wonderful. He's a sophisticated gentleman who can do the weirdest things with a strange face, because he doesn't always know what is normal. It makes him entertaining that way. He can also be dramatic when in art mode.
Makoto: She used to be played around like a doll by authority, but HER AWAKENING IS SO COOL! She's the smart mother of the group, who always let's Ann when Ryuji is being a perv. She's likable because she's a no-nonsense girl who is a leader and has a future ahead of her.
Futaba: She is best girl in my opinion, again, for personal reasons. She's likable because she's a nerdy splatoon girl who is shy, but also speaks her mind. She is lively and just adorable.
Haru: I don't see how anyone can dislike her, SHE'S A PRECIOUS CINNAMON ROLL THAT NEEDS TO BE PROTECTED. She also has another side to her, which makes her all the more better. Actually, now that I think about it, I can't decide who's better, Futaba or her.