Visually, I love deer for spirits (too bad it's not the best pick at competitive play), then squirrel, raccoon, lizard, flamingo and bird all look very good too (although that can be said of the game in general).
Mechanically speaking, I find ray the most satisfying to use, because of the way it builds up and how it requires creative thinking in order to place your tiles in the most efficient way. Ladybug can also be quite addictive to play because of the way you scatter your animals across the board (same goes for yellows when you have the butterfly spirit).
But more importantly than any individual card, is the way in which all those different cards come together, both visually and mechanically. The pleasure of this game is in creating a nice looking environment where all the different parts fit into each other nicely, which in the best cases, can make for quite the majestic results.