I prefer the Key Arts from all the packs where you can see animals clearly before they switched to the one visible animal and the rest are silhouettes format. Pre-silhouette format, I think my favorite key arts are South America, Europe, Tropical, and Oceania. My favorite key arts with silhouettes are Wetlands, Grasslands, Eurasia, and TBA pack. I should also note that this is based on scenery/ aesthetic, not on which animal is featured as the main art on the cover. If I was going by favorite animals featured on the covers, then it would be base game, Africa, South America, Twilight, Arctic, Southeast Asia, and TBA pack as my favorite as well as Grasslands for having a silhouette of a Maned Wolf even though the featured cover animal is an armadillo.
My least favorite key art is the one for North America because that yellow cover is unbecoming, a sea lion doesn't live in the same environment as a moose or beaver, and the beaver is too uncanny looking. After North America, Southeast Asia and Arid's key art are my other least favorites because they're too boring featuring only one animal (although Arid's debatable since we have 2 conflicting key arts, one of which includes 3 animals instead of only 1.)