First playthrough I usually do looks since I dont want to go and look up stats and mvoesets of new pokemon in my first playthrough.
I tend to have Fire/Water/Grass Core in msot of my teams for some reason, after that If Flying wasnt covered, I would get one of those and than just go nuts with whatever types I want. However, I always have a dark type, cause dark types are awesome.
I usually restart my game when I feel like replaying the story over again. On my first playthrough, I used to catch every Pokemon family, but from Pokemon X & Y onwards I'm only going to catch the new Pokemon I think I really want to use straight away because they look interesting to me to use alongside a few older Pokemon.
Any playthrough after the first, I try and use teams I haven't used before. Some Pokemon in the team may be the same a lot of the time but not every Pokemon on the team are the same on each playthrough.
On each playthrough, I generally keep my starter and one other Pokemon out all the time and rotate the rest of the team.