I play mostly AD champs. MF, Yi, etc. I feel that Thornmail shuts down AD too easily. I might be wrong... Is there a good counter? FoN, Banshees?

get magic resist. the fight between tanks and AD heroes is always about countering what items each one gets.

Lots of Lifesteal I guess.

The best way to counter thornmail is to buy a thornmail. The reflected damage reflects back at 30% infinitely.

so your asking how to counter a counter...

lifesteal, not magic resist

Lifesteal.

It's magical damage, so a cheap MR item would help. For example, Merc Treads. Cheap and easy with solid MR =)

MR honestly..