Let's look at what Frozen Heart does:
+100 armor
+400 mana
+20% cooldown reduction
Passive Unique: Reduces the attack speed of nearby enemies by 15%. (700 range)
and what Randuin's does:
+500 health
+70 armor
Passive Unique - Cold Steel: When hit by basic attacks, reduces the attacker's attack speed by 15%.
Active Unique: Slows the movement speed of nearby enemy units by 35% for 2 (+ 0.5% armor) (+ 0.5% magic resistance) seconds. 60 second cooldown (500 Range).
and what Thornmail does:
+100 armor
(Thornmail) Passive Unique: On being hit by basic attacks, returns 30% of damage, before any reductions such as armor, as magic damage.
In terms of sheer tankiness, Randuin's is probably the best individual item. However, the utility of the item is what draws me to Frozen Heart when I play a tank. It gives CDR, which is good for getting peel abilities back up sooner. Importantly, it affects all the enemies whether they attack me or not. By contrast, Randuin's and Thornmail only help when the ADC attacks me. Ideally, the ADC will be trying to attack everyone else first as long as they can safely do so.