I don't actually have dreamies but I do have villagers I like and would like in my town. The hardest are probably the Tier 5 villagers, since they usually get auto-voided by cyclers and people assume there's no interest in them.

Finding Freckles was really difficult since I'd always have to wait to be lucky enough to find people who were willing to take villagers out of auto-void or who were advertising that she was up for adoption. Unfortunately, when I found them, I'd usually have a full town. It doesn't help that I have lots of issues connecting to others via wifi. It took me forever to find her.
The one who's next on my list and that will be probably be equally difficult is Tabby.