If she wasn't one of my near-originals (she was the first to move in after Pawnee was established) I'd say Tammy, but since I've always had her she's not what I'd consider dreamie material (I've never had the requisite dreamie longing for her, especially since I knew nothing about Animal Crossing when she moved in).
Someone I've always wanted is Goldie, mostly because I really, really want a dog irl, and I'd really, really love a golden retriever, but that's not going to happen, so having one come and live in my town is the next-best thing.