I do like a full spread of personalities, and ideally not 3 of a personality, but we'll see.
The second thing is character design; while I like villagers that are cute, I'm also very fond of villagers who are high-concept (by which I mean, have a clear concept that's easy to describe), or whose designs help me flesh out their characters in my head. I don't sweat popularity too too much.
From these two things, I tend to, when possible, assemble a "short list" of villagers I'm interested in, and from there, check off that short list with in no particular order: Aesthetic Theme, Availability, Species Variety, House.
Additionally, I feel like how I feel about them as I play with them is important; early villagers stand a pretty good chance of creeping up into my "favorites" category, ala Hazel or Lopez or Cyrano or Blanche, all of whom I loved (and sometimes lost) in New Leaf.
Being able to build narratives about them through any way, whether design, house, history in my town, is really important. In New Horizons, I'll add "Do I know what to do with their yard," which turned Pietro and Merengue from "short list but not must-haves" into "Well I guess I am married to them now."
At the moment, I'm not concerning myself with a permanent list of dreamies, but I am keeping my short lists in mind.