I like ZUNart, with a few exceptions (mostly the recent stuff, actually). I think it's got charm and fits the game's atmosphere, especially in IN. It's good art in that it is aesthetically pleasing, but it's not particularly skilled.
The expressions are wonderful, though. Also, that hinaface is adorable.
I also think IaMP/SWR artwork is lovely (Yukari's weird hair in her defeat portrait aside). Half of the appreciation comes from how it actually makes the characters look the ages I imagine them to be, for the most part- young Reimu and Marisa, Sakuya being youngish but more mature looking due to her countenance, mature Yuyuko and Yukari, etc. The other half is that it just looks really great, fits the atmosphere of the game, and lends weight to the dialogue for the most part.