Whichever species you like best that has 10 villagers? XD
I don't know, without any leads it's kinda hard to give advice. What kind of town are you aiming for in style?
I voted for penguins because I like the penguins a lot, and I've never seen somebody do a full penguin town.
(There was one person on tumblr who was working on one and had 8 penguins but they ended up resetting their game and doing something totally different)
Go for a specie where it has more than 10 animals. Or you will be doing villager maintenance every day. Trust me I make sure my all deer ping me everyday.
There's a good amount of Cats/Dogs/Deer/Squirrels, but I feel like they're really overdone. I would also go for the Penguins. Plus there isn't one I particularly dislike, guess that's a matter of opinion though :3
I haven't seen an all squirrel town, that would be neat. I'm doing rabbits myself, what's nice about it is that you can get one of every personality. ^^