Hello and welcome, This is my first guide ever, so any comments (good or bad) or always welcome, afterall, we are all here to learn a thing or 2 ^^ i will do my best to make this guide as complete as possible, but since its my first, dont expect to much. first of all: As you can see, i build Rammus as a solid Armor tank, with minor Magic Resist. why? couse i believe he's defensive curl will grant him enough MR for the focus he will get in Teamfights. HOWEVER! this means that this build is only good vs a heavy AD team, since to much AP will rape you in a matter of secconds.
as mentioned before, i build rammus on Armor runes with MR/lvl. ofcourse, here you have alot of choices. you can go with Health quints instead of armor, or attack speed, or ... but for me the armor runes work well for my W(Z) early game.