My opinions will probably change thanks to the upcoming update, but as of now:
Personality-wise
Label is probably my favorite. An NPC that has an actual friendship mechanic integrated? Sign me up! I liked how she slowly overcame her natural shyness and is now a friend that is so happy to see me. And the fact that this is also reflected on Sable's dialogue? Superb. It's funny to me how so many people ignore this trait, considering how much they complain about lack of development and whatever.
From there I also really like Redd, who has some nice dialogue about his fakes and is helped by being tied to a yearly event, Pascal and his varied deep takes about the world and Flick and C.J., who have their personalities shine through both their dialogue and actions, such as Flick making these big bug models (even including light on the firefly) but making only small fish models.
Honorable mentions to Celeste and her cool star signs dialogue and both Gulliver and Gullivarrr because the concept around them is just way too good.
Leif/Kicks/Saharah/Wisp/Daisy Mae are charming, but they are mostly standard "shopkeepers".
Usefulness-wise
It's a bit hard since all of them are useful on their own way, but I have a soft spot for the ones that offer a mini-quest. Doing a little errand is a nice break from the standard play, so Wisp, Gulliver, Label, Gullivarrr and C.J. are ahead on that regard.
On the other hand, their stock varies wildly. Wisp is probably one of the most useful as one of the only sources of color-variants and since he can be found weekly (and even more times), he's a bit ahead of the others. Redd has also color variants and the artwork, all fake/haunted/real is really nice, so he's up there. Flick and C.J. provide 160 unique furniture items exclusive to them and they are also excellent money-makers. Gulliver/Gullivarrr, Pascal and Celeste offer less items but they are pretty unique and very nice to collect 'em. Leif/Daisy Mae are useless most of the time, but they do provide very unique stock (bushes, turnips, etc.).
While they also offer many unique items, I think the stock of Label, Kicks and Saharah is the least useful as they are easily replaceable. I can't get all the nice backpacks from Kicks anywhere else, but I can get a cool Nook Inc. knapsack to scratch that itch. I won't have the pretty animated wallpapers from Saharah, but I can still get 300+ other options from the Cranny.
Excited to see if they have any change on the 2.0 update and also excited to meet the new visitors such as Katrina and Tortimer and the role as a visitor from Reese & Cyrus. It's very probable that the alpacas end up pretty high on both personality (thanks to the Wedding season) and usefulness (which in turn will make Wisp and Redd less useful).
