I play rammus as a tank. This is how i build him my way and i have success with it.

first off u HAVE to get thornmail first. it returns 30% of damage back at enemy unit. next i get ninja tabi. with rammus's passive ability the added armour helps and u get a dodge chance increase. Next go for a sunfire cape. if you are running out health consistantly get the giant's belt first. The added magic damage per second is good when you get an enemy with taunt. Next go for a frozen heart it decreases enemy speed and gives you alot of armour. Go for a warden's mail next it will increase your heath regen and give you more armour. last save up for a warmog's armour. it gives you alot of health and health regen.