*UPDATED*
For Rammus, the best way to go is by HP and Armour, for his passive converts his armour to attack dmg by 25%. also that his defensive curl ball depends on armour for extra damage and increases armour as well.

so i made it be for the rune build, I got greater seals of resillience that would benefit the defensive curl ball and give a little bit of base damage so you get a better start to the game. the greater glyphs of warding will help with your magic damage taken, since your more vunerable to magic then normal attacks. the greater marks of alacrity will help with your normal attacks to finish them off. the quintiessence of defense will help towards the end, which is where you need your strength the most. the quintiessence of alacrity will further help your attack speed to finish off those that run alot, but not fast enough.

for the item build, depending on what your up against, then the second last item should be guardian angel. if you want, change the boots to boots of swiftness so you can chase better and run better, but you may need just that little bit of dodge to help.the thornmail gives you good Armour, thus more resistance+defensive curl damage and also return some damage even if your not in the defense curl. the sunfire cape will make damage around you per second so that will help (not by much though) and will give you good hp+armour. the warmog's armour will help your survival drastically and add more then 1k hp. the randuin's omen will give you 300 hp and more armour then sunfire cape, thus helping more with your attacks and taking alot more hits then all other champions.

Early Game:
just try to farm (but of course try to get first blood before doing so) as much as you can so you can get a fast start, once your taunt is up to level 3 should you start team killing.

Middle Game:
make sure you keep combining your defensive curl with your taunt so they don't have a will to not attack you (thus making them take max dmg until the taunt finishes). use your rollout attack only if teaming or chasing down an enemy (don't run away if others try to stop you, as long as your team mates are on the way, you'll go fine). and when using your taunt it's a perfect time to use youe ultimate, since they always run whenever you don't taunt them.

Late Game:
by now you should have killed more then your deaths (or have more assists and kills combined then death). as you keep doing those combinations, you will be able to assist in team fights alot and can help you solo almost every champion in a 1v1 (i haven't versed all champions in a 1v1 yet, but i'm pretty sure you can go up against any)
*NOTE* there are some builds rammus cannot compete with because of penetration.

Post a comment on how this works and any other questions regarding what i've set/said