I think overall, the cats would be the best in terms of variety with the cat villagers that already exist. I have two cats currently. I do really like the frogs ( I have two), but some of the other frogs are scary! I also really like the deer species and have two of those in my village as well.
You can't really choose villager species that have 8 and below characters so I would probably pick sheep. I love the sheep a lot! Imagine a town filled with little fluff balls.
But the 9 villager limit aside, Hippos or Anteaters definitely!
I guess cats, if I could expand it to include all types of cats (domestic and big cats).
Truthfully, I don't know that I'd want a village of all one type, because I love seeing all the different animals. There are some that I'm not a huge fan of (just can't get behind the gorillas), but right now, Salem has a bear, a bird, a penguin, two kangaroos, a bunny, a bull, a squirrel, and an elephant. They're all so much fun, I couldn't imagine not hanging with any of them.
Squirrels or frogs! I want to buy a separate copy of the game just to do that ;_; I had a bunch of frog amiibo cards and I was very close to putting almost all of them in my town.. but I had to stop myself because I have other villagers that I want more than frogs.
Monkeys since they are underrated tbh. Although, there are only 8, and there are no smugs. Thats a problem of why they arent that picked. I might fill in those two extra spaces for gorillas if i were to do it, but its not likely ill make a one species town wholly than monkeys.
I find cat, deer, dog towns boring because they are overused but thats just my opinion. Maybe one species only town overall is boring because there isnt such a diversity of the bitter and sweet. Villagers arent always about their beauty. Its also about their preferences. However if you wish to be living in a place with beauty only, suit yourself. Im not stopping anyone from making one species towns. But i wish there is more than just being "normal".
Youre talking to a person with 6 monkeys, and 4 high tier villagers of other species in their town so its kind of ironic. But you know, theyre monkeys. Theyre one of the least liked species in this forum. And i was always this oxymoronish person, so yeah.