So hard to choose, but I'll got with Redd (Tom Nook comes close 2nd).
I just love Redd's design and I like to imagine him traveling from town to town trying to rip off people, and getting into trouble hahaha. I liked him better when he didn't sell JUST paintings though, and was pretty much just a greedy merchant.
I really like Phyllis. She's one of the only really grumpy people in the game, and I always liked her sarcasm when i visited the post office at night. Then one time, I went to The Roost when she was there, and I talked to her, and she was asking Brewster for a coffee to take back to Pelly and I was like bawwww