Squirrelypaws
The original cover of Dawn! Honestly, I am pretty much in love with that cover. I actually bought a second copy so I could put the dust jacket on my wall as a poster. And I have the lower part of the front, with the cat silhouettes looking out over the lake, set as the wallpaper on my phone.
A lot of the art from the guides ranks pretty high for me too. That picture of the three spirit-kits in (I think) Code, Crookedstar's profile picture in Cats/TUG…
And yes, the new covers for the series. I actually hadn't really looked at them prior to joining and seeing the premade avatars using them, but wow are they pretty!