Interior. There are lots like Whitney, Diana, Julian, Marshal, Lily, Sherb, Apollo, Merengue and probably Wolfgang. Exterior, I don't pay much attention. As long as they don't look bad, I'm fine with them..
I'm fond of teal/mint colors, and also of natural aesthetics, so I quite like Lily's exterior! the log posts outside of the house and the wooden window trim look beautiful with the teal!
7/10 of my villagers have Japanese-style house exteriors, so that's definitely my favourite overall look. As far as my favourite interior goes, it's a hard call...
But I think Tokio/Cranston wins the "Coolest House" award for my lineup. The red/white colour coordination is gorgeous, and the Chinese motif is extremely fitting given that (despite being a symbol of Japan), extant crested ibises -- including the ones being reintroduced to Japan -- are actually from China.
The penguins have the best interiors IMO. None of them use normal stuff (they use mostly ice and seasonal stuff) so their houses always look grander than the others. In terms of exterior, I like most except for the metal looking ones which are colorful and gaudy.