personal experience, played a malphite game in which i had 540 armor and a thornmail.
Lucian and friends who were mostly ad still wreak havoc on me. Mind you thornmail returned 107 damage + 15% of the incoming from lucian. He still deals nearly 2.5k damage in a few seconds.
Is it better to just stack hp items with one armor item in build? Or am I missing something.
People here aren't saying it clearly... but I can help lay it out some. I put a lot of work into figuring out some of this for runes for my own build and found out one useful bit of information:
Armor in LoL doesn't give a flat bonus. It actually reduces the amount of damage you take based off your HEALTH and ARMOR. Building armor is great, but unless you are building health too at a rate of (personal estimate here, but seems to be good) of about 750-1000 health per 100 armor. It also helps to build up mana and MR because you can build 
, 
, 
and 
and end up tanky as hell. If the enemy gets a bunch of armor pen that is making you take more damage then focus on health and not armor.
Since armor is based off health you NEED to keep a high health with it, armor alone will not help. Using armor alone is why people complain tanks melt. Build a ton of armor and HP and you will be really tanky.
Edit: http://leagueoflegends.wikia.com/wiki/Armor for more information
armor by itself isnt enough you need armor and hp
vs heavy ad comps that have dps or high attack speed items like
helps greatly to reduce damage taken
its really more of how ad interacts with auto attack.
100% ad ratio on every auto.
250% with IE and crit.
game been poorly designed around auto attackers and crit and ad.
does it matter how much resistance you can get if the enemy ad damage attack speed multiplies to ridiculous amounts.
I think you have to look at what peoples roles are, you're a tank yes but ADC's are supposed to shred tanks and objectives whilst being very squishy, you are required to tank some of that damage but usually for doing so it should give your Mid laner or jungler/top laner to get on them and remove them from the fight and if they do so, that's where your tankiness will benefit the most.
If you're taking this damage without the opportunity for your assassins to burst the ADC then you're taking unnecessary damage.
ADC kill tanks Assassins mid/jungle kill ADC's Bruisers are a half way between the both and deal damage and have tough skin.
I hope this helps :)
Don't listen to those guys. Tanks are not effective because: Juggernauts can do same + sustain (try to kill Darius). Most armor pene items are core for most AD champions.
So yeah, armor sucks this season.
the effects of armour diminish the more you have, so if you had 300 armour (75% dmg reduction) to your 540 which would be about 85% at most I can imagine. It's far better to stack health after the 270ish armour range
You forgot to mention that you dove enemy team 1v5 and their adc jungler mid top are attacking you at the same time
The problem is ADCs are out of control and Thornmail is garbage now, for the AR/MR vs HP thing, only aim for 220AR/MR and 4K HP.. After 200 AR/MR it really isn't worth build it anymore and you get a nice 70% resist from it.. after you get to the 200 mark just get a Warmog and go ham.
has 
he literally gave up lifesteal to counterbuild you since you had a lower HP build going two armor items w/o hp additionally 
both had and zed had 
that's a lot of armor pen.
So yeah with that much armor pen on their comp you needed more hp.
Also you were playing SUPPORT 
thus behind on the gold and level curve.
people do not understand that being tanky does not mean you can just tank a late game adc for 5-10 seconds. if an adc is free firing on you, they are going to melt you. lord domink's regard cuts through bonus armor at 45%. so like you effectively have only 250-300 armor to that lucian which is still quite a bit but not going to make you this unkillable monster to Lucian. especially since the other part of lord dom's is increasing his damage overall to you by 15%). If he further has a cleaver and crit on top of that well you are now losing another 1/3 of that armor. so 75 armor? you are now at around 175-200 which lets be real, you are going to be cut through by an adc at that point.
this is why tanks go down so fast to adc's late game. because adc's have the one item in the game designed to cut through a tank. if an adc gets to free fire on you, then your team failed to get to the adc which is a key part in winning team fights.
being a tank does not mean you can just blindly walk into the enemy team's carries late game and then soak up all their damage. its to ensure that they have to focus you down or you will be a menace.
Tanks are actually at the strongest in the mid game when they have two armor items and the ads don't have their pen items yet, and no one can deal with their tankyness since they simply do not have the items yet.
