If I would rank the guardians as a whole (design and lore), it would be:
1. Tapu Koko (I like the tribal warrior look and it's fickle nature. I'm sure with Koko it will be never boring)
2. Tapu Fini (I expected it being mysterious, akin to a sage. But I love the idea having Fini inside the shell, which reminds me on clams, pearls and mermaids and I love mermaids)
3. Tapu Lele (CUTE! I love the teapod shell, I can see it as the innocent child of the Tapus while wise. But I wonder how Lele would be as character. I'm expecting it as being shy as a counterpart to Koko)
4. Tapu Bulu (Somehow I need time to get warm up with it's design, but again I didn't care at much for it as for Lele and Fini. Maybe if I see it as the Gentle Giant with a green thumb and encounter it, maybe I can warm up with Bulu)