Mimikyu, no contest! One of the most adorable Pokémon to ever exist, and a lifesaver in some of the toughest battles in the game. Who would have known a Mimikyu could save the Alola region from eternal darkness just by snuggling up to Ultra Necrozma?
Other favourites of mine are the Litten line (which I chose as a starter in both of my main playthroughs because I just couldn't go with the others), the Rockruff line, Togedemaru, Type: Null and Silvally, the Poipole line, Necrozma itself, Marshadow, Magearna, Zeraora and the Meltan line. They really outdid themselves with the Mythicals in particular this generation. And if regional forms count, I think the Alolan Sandshrew and Grimer lines are amazing, as is Alolan Raichu (even if nowadays I prefer the original Raichu).
Other favourites of mine are the Litten line (which I chose as a starter in both of my main playthroughs because I just couldn't go with the others), the Rockruff line, Togedemaru, Type: Null and Silvally, the Poipole line, Necrozma itself, Marshadow, Magearna, Zeraora and the Meltan line. They really outdid themselves with the Mythicals in particular this generation. And if regional forms count, I think the Alolan Sandshrew and Grimer lines are amazing, as is Alolan Raichu (even if nowadays I prefer the original Raichu).