Don't be scared by the old aspect ratio.
Lovely Complex is considered one of the most universally loved shoujo series, and it, surprisingly, passes the test of time. The series that was released in 2007 is still relatable, tackles some topics that will forever be relevant, and looks quite nice even by modern standards.
The story follows a group of school kids, one of which is Koizumi Risa, a really tall — 172 cm — and loud girl. She's quirky and sarcastic, but kind, cheerful and honest and the only thing she wants is just to be happy. The other kid is Otani Atsushi, who is a short boy — 156 cm. He's as loud and crass as Risa, maybe even rougher around the edges, and at first they don't hit it off well. Their relationship is filled with creative bickering which makes everyone who witnesses their verbal fights view them as a comedic duo.
As time goes on, they grow to tolerate each other. They have a similar taste in music and a sense of humor, they realize they enjoy each other's company and become friends. Who still bicker and argue, but with no actual malice behind it.
They went through the "enemies to friends" stage, but now they're entering the "friends to lovers" part of the relationship development, and that's where the word "complex" in the title starts to shine. Their physical differences make them question the romantic part of their interactions. Otani has trouble with girls because they never see him as someone they can give a chance to: he's cute, sure, but he's not boyfriend material. So it takes a lot of effort for him to accept that he can be loved despite being tiny. He can be loved for his empathetic, albeit fiery and mischievous personality, for his kindness and inability to give up. And Risa goes through some introspection, too, that is not only related to her height, but also to her perception of other people.
Sure, the 4:3 ratio and animation style look a bit outdated, but this series has been made with attention to detail. Our characters are in Osaka, so you can experience them talking in the Kansai dialect. Risa's always wearing different clothes and pairs them with different hairstyles, from buns to curls to pigtails and everything in between, and it takes a lot of effort from the animation point of view to create different looks for a character. The whole story follows our character for a quite prolonged period and doesn't feel rushed. The development happens organically and believably, and the pace feels right.
And the topics that the show brings up are always relevant. So, if you need some uplifting romantic story that is centered mostly around characters overcoming their insecurities, then Lovely Complex is your choice.