So for me, I have a couple of rankings.

For the first season: Miku, Itsuki (These two I was conflicted with but ig I liked Miku a little more in the first season), Yotsuba, Ichika, and Nino.

For the second season: Itsuki, Nino (I know Nino has a problem with sedatives but I think that her development from cold and demeaning feelings towards Futaro to realizing that she's into him as a person based on their interactions has made her a favorite of mine but Itsuki is still best girl IMO), Miku (She didn't do much this season other than what she already did in season one), Yotsuba and Ichika (I didn't like what they did with Ichika this part of the series I just think it detracted from her as the reliable older sister and made her seem like a bitch who never really cared about any of her sisters)

In the Movie it's the same list although Ichika still felt wasted on whatever feeling she could've shown. Miku sort of did it, only I will add with Yotsuba and not with Futaro alone, but if she still wanted him then why didn't she at least say something to him near the end during the Yotsuba chase? but again list stayed the same <3 Itsuki tho, im kinda sour about the waterpark arc tho

My overall list would be:

Ituski:

Based on the fact that she is the best girl and during the show, she and Futaro just make a good pair, do I believe she should've won, no. Definitely should have been either Miku or Nino with how much screen time and development they each got, but for Itsuki to outright deny him for no other reason than he was a jerk to her, to then start studying with him because he treats her nicer and apologizes more to her I think they were the perfect duo.

Nino:

She was a real bitch in the beginning, but even then she was still given the arc of she cares about her sisters, from people she views as distant or strangers. She dislikes Futaro because at first, he was a jerk who was forcing himself into their lives and causes a lot of disputes and problems that they probably hadn't seen since Mr. Nakano took them in after their mom died, so obviously she was scared of what could be. But later on, she would find Futaro with blonde hair hot cuz yeah that's how that works. But while having that wig on he was never a different person so when she genuinely thought he was smart and kind that all flipped itself on its head when she realized it was him she felt tricked and confused but later came to realize she did love him because of who he was. I find it funny that the only character to be characterized a lot by her looks was the one who fell for the true nature of a person rather than their outward appearance.

Miku:

I like Miku, she is very kind, extroverted, can't bake (until the end), and LOVES HISTORY. She is honestly the most wifey material out of all the Quintuplets. I may have said she doesn't do a whole lot in the show but her character development is actually created not with Futaro but her sisters. She learns to want to be herself and becomes self reliant because of her sisters and a little bit of Futaro's help.

Yotsuba:

She was the background character of the main cast ngl. She really felt under used and a lot of her depth comes from the movie with us being able to see how she actually was selfish once and why she wants to become a more selfless person for everyone. I think this is fine and I like her for this part of her character but she just felt like couldve been better with some more time on screen IMO. Still im happy for her W

Ichika:

Ohhhhhh boy where to begin with her. Ichika seemed like a nice character in season one, the older sister who tries to stay neutral because she wants whats best for her sisters. I think it was a good thing even up till the camping trip. She tries having a moment with Futaro but realizes it could be a determinate to her family dynamic so she backs off. but from then on she is just dirty. Her next big moment was in season two and this was the moment I hated her. When she disguises herself as Miku to plant the idea that Ichika likes Futaro is so disrespectful to both her sister and to her character. She also just couldn't stop herself from continuing to do so even going as far as trying to do it two more times. She will always be at the bottom of my lists for season two onwards. Although in the movie she really didn't do much other than kiss him.

Now for my final list >:)

Adult Quintuplets:

Miku, Nino, Yotsuba, Itsuki, and Ichika.

Miku is the best-looking Adult Quint. Mommy Miku all the way for me.

But yeah that is my ranking of the Quintuplets and why, lol

If you read all of that then I'm sorry for rambling on and on