My favourite house interior out of my current villagers is Blaire's, she has a bit of an odd mixture of furniture but I think it works really well together, helped by the fact many of the items stick to a flower theme. I also like how it doesn't go over the top with colours and mostly only adds some purples and reds to brighten up a relatively plain interior, creating a bit of an elegant interior despite the mismatched furniture/item choices.
Diana is another villager of mine who has an interior that I like, she also has a mixture of furniture sets in her house that work well together and look quite classy which suits her character, the deer decoration is also a nice touch to match her species and background. I slightly prefer Blaire's interior though as it seems more homely to me.
Daisy, who I no longer have on my island, has a lovely interior with well matched furniture, in a nice colour scheme, that makes her home seem very cosy and relaxing.
Special mention to Cheri's interior, I've never actually had her on my island but I think she also has a very cosy home and I like the fact that it feels like a practical interior with a bed, kitchen area and bathroom area, plus the colour scheme really matches her character.
(pictures are from Nookipedia to show the proper interiors, most of my pics of the interiors had extra items in them)