My item build on Rammus is Philo Stone, Heart of Gold, Tabi/Treads (depending on enemy team), Warmogs, Thornmail and Guardian Angel. I get regrowth pendant as my first item for easier laning and the two gold items are there so I can stop farming and be more useful in the team.

Warmogs is my first “big item”, because it gives more health than anything else, which is what you want when you play a tank champion. While I agree with the consensus that armor items are highly effective on Rammus, you really need to have a decent amount of health before you can start stacking armor and magic resistance. Warmogs and Thornmail when taken together allow you to defeat almost any physical DPS champion with ease, and finally, Guardian Angel is purchased because if for some reason you manage to get yourself killed, coming back with 40% health restored is often enough to turn the tide of a teamfight.