I guess the Swords of Justice. It's the only trio I can think of right now where I don't dislike any of the 3. (Cobalion is my fave though)
Well it is kind of a 4 way tie for me the Legendary Birds and Legendary Beasts because of nostalgia, and then the Aura Trio and the Light Trio. I will say why the sun and moon and the Ultra versions of the games are not my top favorite game wise I do absolutely Love Salego and Lunala and I like there combined forms with Necrozma