in every game the team has two tanks you should go your crit items first right into your armor pen item 3rd because if you don't you aren't ever going to kill their front line and it doesnt matter how much damage you to do to squishies.
this applies to ALL metas.
even if the enemy front line aren't true tanks, but bruisers who have an armor item or two, still build it. like lee sin jax building mostly tank items after their primary damage item? LDR.
grievous wounds is also hideously underrated by lower elo players. if you are against maokai and you say "wow holy shit he is healing for so much" fucking buy your executioners. it doesn't matter how much damage you deal to him if hes gonna heal so much that he lasts a few more seconds.
generally speaking here's how it works
1.) tanks who have a heal (gragas/maokai, etc.) or swain/vlad/darius/mundo (or a soraka) types = Mortal Reminder
2.) standard tanks and bruisers who don't rely on healing and no other major healing on the enemy team = Lord Dominik's Regards.
Every game you will have to get one of these, and if you already got your crit get it 3rd. waiting till 4th or 5th item doesn't really give you nearly as much value. i mean you can build another zeal or crit item, but you still don't do that much damage to the only people you can hit. you can go bork or another lifesteal item but you won't get much value out of it since the passive is physical damage which is reduce by armor. you can build a defensive item, but it doesnt matter how long you live if you can't kill the front line.
whoever kills the enemy front line first usually wins the fight because then your team can fuck up their back line. so build to kill the front line as an adc. (unless are the broken piece of shit that is twitch and can flank the back line. )