Me and my friend wanted to see how much armor you could get on Malphite. With about 3 thornmails, a ninja tabi, and his w (shield) up he gets around 580. This equates to 85% + 10% from basic attacks.
With 6 thornmails and the right runes and masteries it would probably be over 900. With 5 and a ninja tabi you could probably get over 100% reduction on basic attacks.
EDIT:
Quick calc on leaguecraft puts the 6 thornmail + masteries + runes + w = 1051.134 armor (91.314% reduction). This does not include any auras or buffs.
5TM + Ninja Tabi + others = 946.134 armor (90.441% reduction 100.441% on basic attacks). I wonder if this means that basic attacks with no armor pen will heal you.