In my first playthrough, I just usually use whichever ones show up first to me in the wild--if I end up liking them, they stay in my team. If not, they get boxed, and I continue my organic search for Pokemon that I like. I typically play blind now, so I come into a new generation without knowing much about all the new Pokemon or the new forms that older Pokemon can take.
During my second playthrough, I plan my team out beforehand using only Pokemon available in the game (for example, I won't be using Mareep in a Sun playthrough), and since I have access to all the Pokemon from my first playthrough, I can easily trade them over as eggs and start my game that way. I always use my starter in both playthroughs, however, and I must always use a different starter each time.