Hana Yori Dango.
That manga title still rings in my head. I had just finished it and it was like I was suffering from a manga hangover. I felt like I was sucked in their world.
It all started... I dunno, sometime when I just started high school. People here got crazy about "Meteor Garden" and I was like, "Hah?" I was no fan of TV soap operas especially subbed ones (and it was dubbed here in my country), and I was a die-hard "Sub it, DON'T DUB it!" person. I had absolutely no interest in Meteor Garden, and I kept telling myself, "Stick to the original, Hana Yori Dango!" But I had a problem: I can't watch the anime due to lack of resources, and I still wasn't stuck in the world of manga. Then they started showing the HYD anime here (dubbed) and I got sick of it so, after a long long long time when I was finally introduced to the manga world, I started DLing it like mad and read it in a 3-sitting marathon.
So, here I am now, still enthralled and in love with it. It's already shown in the other comments, but the char dev is good, probably the best I have ever read (and I read a lot of manga). At first it's hard to believe how Tsukushi could live her life that way (and still fall in love with one of her previous bullies), but that shows so much of her good side, how she could be so strong in herself.
The way she handles herself is admirable and down-to-earth. I'm alive, I'll just be me. The way she gets confused or scared or angry shows the readers how she is just a normal girl living in Tokyo. This is her life, she breathes, she feels, and she has a personality that isn't just any normal 2D type but is a very real 3D. The same goes to the other main characters.
Sure there are some aspects of the story that seems unreal, but this is what makes the plot run, isn't it?
Reading the first few volumes made me cringe because of the art, but like any long on-going manga, the art improved dramatically, making you feel as if the way they look changed as you got to know them more.
If you're a first time reader, forgive the art and get on with the story. It's very likely that you won't be disappointed.
I just wish that the ending was a bit different and wasn't a cliffy. Though I guess this is what tests how well you know the characters, no?