I honestly don't like Ginta very much at all. Yuu is definitely my favourite of the two.
Marmalade Boy was one of the first series I discovered and loved when I first got into anime and manga. It's so bizarre and cute. ^^; It can be a bit dramatic for my taste now, though (but I still enjoy it, anyway). Meiko's my favourite character... she became my favourite character immediately. I think it's a combination of her appearance (mainly the wavy hair) and the fact that she is calm, intelligent, and sophisticated... she reminds me somewhat of my favourite anime/manga character (Kaiou Michiru, Bishoujo Senshi Sailormoon). Plus, I find her affair with her teacher interesting (and at the same time, very sad).