The only real guarantees for me this generation are Primarina and Alolan Ninetales, though the latter will probably be a little more situational, given that they both share the same secondary type. But the beauty of the main story progression is it doesn't specifically require you to have a team strictly diverse as online battles tend to require, given that the Gyms have in previous games kinda structured our teams in the past. Something I know I've always done is at least made sure I've had something to take care of an upcoming threat.
Some possible contenders are Midday Lycanroc, Lurantis, Ribombee and Sensu Style Oricorio, but I think this Generation I'm going to see where the wind takes me in terms of what ends up being a part of my main team. Something I never really did with any of the past generations is get a feel for what the Pokemon that come with the generation are like. Even now, there are a number of Pokemon I've never used, despite working through X & Y with a full box and party of Pokemon to switch between when progressing through the story.
There are some really interesting Pokemon in Sun & Moon, and I'm looking forward to getting my hands on the games to see what they are all like. Fingers crossed the Alolan forms are all they're cracked up to be~
