In 5e it's actually called an Opportunity Attack. AOO is older editions.
You're in a combat with a Lich and two zealot acolytes; you know the Lich has some pretty nasty effects up its sleeve, including some very high level stuff. The zealots, you know, mostly cast minor buffs and create chip annoyance with cantrips. You've already rolled initiative; the turn order goes: You, Zealot 1, Zealot 2, Lich.
How do you 'seriously plan your moves ahead of time' so that you can tactically and intelligently respond to the situation and counterspell only the enemy spells that are most important to counter, without wasting your reaction on counterspelling a zealot casting shield of faith, and not having a reaction to stop the finger of death that comes later in the same turn?
You seem to be implying that you can and do and others should, without the game asking them when or if they want to use a particular reaction when it becomes available - could you explain your reasoning and method to me?