In Ultra Sun the list of Totem Pokemons you encouter are as follows
>Gumshoos(Lv.20)
Marowak (Lv.25)
Lurantis(Lv.30)
Vikavolt(Lv.35)
Mimikyu(Lv.40)
Ribombee(Lv.50)
Their respective types are
>Normal
Fire/Ghost
Grass
Bug/Electric
Ghost/Fairy
Bug/Fairy
From you can see above the Totem Pokemon theirtypes and Level.
The Alolan League
>Molayne(Steel)
Olivia(Rock)
Acerola(Ghost)
Kahili(Flying)
Incineroar
Pros
-Solid HP
-Excellent base Atk stat
-Reliable Def and SpD stat
-Gets good moves
Cons
-Low speed
Decidueye
Pros
-Excellent Atk stat
-Good SpA and SpD
-Fatser than Incineroar
Cons
-Pretty low Def stat
-Less base HP than Incineroar
From the above information we can conlude that Incineroar is the best as it almost covers all the types which we will face through the game and theother are covered with the moves it learn.
So we will go with Incineroar.
Incineroar@Incinium Z
Ability:Blaze
-Flare Blitz
-Earthquake
-Darkest Lariat
-Fighting move(Double Kick,Brick Break,Low Sweep)
Flare Blitz for STAB and has a very good Atk stat.
Drakest Lariat for STAB as well as Z move.
Earthquake for higher type coverage.
Fighting mob=ves give even more coverage also aginst Inci's weakness rock.
You can use flame charge if you want to be fast.
Also Thunder Punch is great coverage and physical to hit those Water type out
Using the above moveset you can counter every Totem Pokemon and the Elite four,your rival,The Champion
Hope I Helped