Easy! Top favorite: Serperior.
As for a list of my favorites? Well, in no particular order (and from a long list I just have copied over from Donphan): Serperior ​(duh), Scolipede, Typhlosion​ (+Hisui), Decidueye, Corviknight​ (+Gmax), Mew, Meowscaradra​, Lycanroc​ (All Forms), Lucario​ (+Mega), Sceptile​​ (+Mega), Ultra Necrozma​, Zeraora​, Reshiram​, Umbreon​, Cyclizar​, Terapagos, Koraidon​, Miraidon​, Eevee (+Gmax), Naganadel​, Salazzle​, Yveltal​, Noivern​, Goodra (+Hisui), Greninja​ (+Ash), Volcarona​, Cobalion​, Zoroark​ (+Hisui), Zebstrika​, Giratina​ (+Origin), Arceus, Garchomp​ (+Mega), Mothim​, Luxray​, Latios​ and Latias (+Megas), Absol, Rayquaza​ (+Mega), Lugia, Tyranitar​, Ampharos, Gengar​ (+Gmax, Mega) and finally Charizard