On my Nano tank, it is definitely my Nem as it is the brightest thing in my tank. In my 40, what catches my eye is all that dang algae but I always make sure the high end stuff is doing good.
My blue sympodium. It's always upset if something is off, after that the God of War Zoas, they're second for letting me know if the tank chemistry is off. After that it's the fish, they beg for food when I walk in.