If you're looking at this from a type advantage standpoint, Rowlet would be the best choice for a Sun/Moon Nuzlocke in my opinion. Here's some stuff to take into consideration.
Rowlet
From a type advantage standpoint:
Advantages:
• Super effective against Hala's fighting-type team. (Grand Trial #1)
• Super effective against all water-type Pokémon in Island Trial #2. (Waterium Z)
• Super effective against all grass/bug-type Pokémon in Island Trial #4. Watch out for Trumbeak in the Totem battle. (Grassium Z)
• Super effective against Olivia's rock-type team. (Grand Trial #2)
• Super effective against Nanu's Krokorok. (Grand Trial #3)
• Super effective against all fighting-type Pokémon (dual typing, dragon/fighting, assuming you have Pluck, Peck or similar) in Island Trial #7. (Dragonium Z)
• Super effective against Hapu's ground-type team. (Grand Trial #4)
• Super effective against Hala's fighting-type team in the first Elite Four battle (Again, assuming you have some flying-type moves), but watch out for Crabominable's Ice Hammer.
• Super effective against Olivia's rock-type team in the second Elite Four battle.
• Super effective against Acerola's Palossand and Dhelmise in the third Elite Four battle.
• Super effective against Professor Kukui's Lycanroc in the final Elite Four battle.
Disadvantages:
• Weak against all fire-type Pokémon in Island Trial #3. (Firium Z)
• Weak against Team Skull's poison-type Pokémon, Plumeria's Salazzle is a total no-go. This is the biggest disadvantage - but if you manage to get your hands on a Diglett, Mudbray or Trapinch, it'll be a breeze.
• Weak against Acerola's Froslass in the third Elite Four battle - watch out for Blizzard.
• Weak against Kahili's flying-type team in the fourth Elite Four battle. Avoid using Decidueye at all costs here.
From a general standpoint, Rowlet's stats are pretty balanced in its first two stages, HP is pretty good and the only real downfall is speed. Decidueye has great attack/sp. attack, but watch out for speed and defense.