Agent S! Goes perfect with Kid Cat, I also tend to like a lot of peppy villagers for some reason, Benga and Bella coming tied in second place. Also Sally.
I actually like so many tier 5 villagers, but here are a few:
Bertha, Bree, Curly, Deli, Dora, Filbert, Gladys, Hamphrey, Maelle, Rocco, Rod and Winnie.
Okay, maybe a little too many.. but if I had to pick one it would be Filbert.
I don't really like how there even is a tier system for villagers. I personally think every villager has their own personal value, and popularity shouldn't really matter.
I honestly can't think of a single villager who doesn't have at least one fan somewhere on these boards. I'm guessing by "popular" villagers you're talking about Julian, Colton, all the deer villagers, Stitches, Daisy and Rosie.
As pointed out before, the rise in lower tier villagers is rising as they're harder to find. Lower tier villagers who I never really thought about but who I think are great are: Sly, Cranston, Penelope, Cyrano and Beardo.