I love Katrina's shop in terms of the exterior look, but I have never actually visited the tent enough to unlock it LOL.... I usually play late-ish but not late enough to generally want night-owl ordinance, so I usually miss her when she's in town
In terms of shops that I have actually stepped inside myself, it's hard to say between Shampoodle and the Nookling shop ... I think specifically the small Nook's Cranny shop is very cozy, but Shampoodle has such a nice vibe, and the second floor landing is cute from outside!
I could pick so many of them, New Leaf main street just has such a good vibe.
I love Shampoodle because I like how it's upstairs from Able Sisters, that just gives it such a cute cozy charm in my opinion. The post office is also a favourite because I love the pelicans. Kicks store also has a really nice design. Leifs little garden shop is super cute but it's definitely too small, he deserves his own big shop and I wish he didn't get sucked into the Nook store.