Personally, I think it something like this
Starter/Primarina - Since the comment from Ame. Could however be replaced by any starter
Arcanine - Major quest
Mystery egg - Major quest
Nidoqueen/Any of Corey's mons - Significance to the story. I usually encounter Nidoqueen first and I think it ties well with Cain's Nidoking.
Absol - Story relevant
Gyarados - The quest spans over such a large portion of the game (Could be replaced by for example Sylveon or, most realistically, certain late-game encounter:
