Pikachu was the BW ace.
Otherwise, why use him over Pignite/Leavanny/Krookodile against Trip's Serperior? As much as we hate how things went there, Pikachu was the BW ace.
So I don't think someone else that isn't the starter or Pikachu for Ash will be the Alola ace.
What are your thoughts now, after the generation? Do you think Lycanroc was the ace?