Rammus in my opinion is one of the best tanks in the game because of his crowd control and incredible durability. He is very simple to play for most people in most situations, but please dont expect this build to make the player. If you are bad at playing rammus, then this build won't work. If you play rammus you cannot be afraid to initiate battles, that is your job and if you dont do it then you will surely lose. You also must communicate with your team. If you running around rolling into things with powerball trying to start fights and your team doesnt know that, your all screwed. Communication is key to winning any game.

Pros

Very survivable
Good CC(crowd control)
Ganking ability

Cons

Not great without mana
Cooldown problems sometime

My rune choices are fairly simple, to make rammus more tanky obviously, and the flat armor and mr makes gives him an incredible boost early game. If these runes aren't satisfactory to your playing style than feel free to change them, although i would keep the basic idea of increasing resistances. Things such as armors/lvl would be acceptable.

Masteries are also fairly self explanatory. I choose to max out my defensive category seeing that we are building a tank champion, and put as much into my regeneration masteries as possible, while still mastering my exhaust and ghost. Defensive masteries are key to maintaining a good tank.

My skill sequence is mainly used for an early game gank in lane. Powerball is a great initiator, as it is rapid, deals damage, and creates a slow. Some would argue that puncturing taunt would work better in early gank becuase it forces the champion to attack you, keeping him from fleeing, but with only 1 skill point into that skill, the taunt isn't near long enough to be effective which is why i get it second, especially on champions that seem to be pushing your tower, because it can bring in some easy kills if they are taunted underneath of it. My third skill is defensive ball curl for obvious reasons. After i have gotten my 3 initial skills i focus on maxing my w and e instead of q, as they contribute the most to your durability and survivability, Although i would put a point into tremors whenever possible.

My summoner spells are argueable, only because i believe that no summoner spell is better than another. I choose ghost and exhaust because ghost can really help when chasing down or running from enemy champions, and exhaust can do the same thing, greatly increasing survivablity. Exhaust also helps tremendously in early game battles. Most importantly, i use ghost and exhaust because that is what im most comfortable with, which brings me to my point. Just because i use ghost and exhaust doesnt mean you have to. use whatever you feel the most comfortable with and whatever fits your playing style best.

Here is the important part. The items that i chose can be argued depending on your opponents, but i feel that they are pretty universal in any game you play.

Null Magic Mantle
This item is a great early game item, especially when your laning against ap. With this, plus your runes, you will have about 80 magic resist at lvl 1, which is ground breaking.

Mercury Treads
Finishing off what you started at the beginning with null magic mantle, mercury treads is a great item, because of its early game magic resist, tenacity affects which can help in sticky situations(lol) and of course the always neccessary movement speed.

Thornmail
I think that this item is an absolute MUST on rammus. It adds a tremendous amount of defense, and an incredible passive that maximized rammus damage output from high attack speed opponents at an amazingly low cost. Although the passive is not always effective against some teams, the defense is required. If there is a champ like yi, xin, trynd, or noct on their team, dont even consider dropping this item. It's a game changer.

Banshees Veil
By the time you can afford this you are probably in need for some magic resist. This is the perfect mid game item for a tank in my opnion. Adding required health and magic resist for any tank, and an additional magic boost which rammus could definately use, and on top of that, a passive that could potentially save your life.

Sunfire Cape
Some people would argue that you need this item earlier, but i dont think thats neccessary. This is a good item to increase your hp and add a good amount of defense as well. With the recent nerf, the passive isn't as affective as i wish it was, but it can help in minion farming if need. BE CAREFUL NEAR TURRETS WITH THIS ITEM. IT WILL TARGET YOU IF AN ENEMY CHAMPION IS NEARBY.

Force of Nature
The ultimate magic resistance item. Adds a ridiculous amount of magic resist, and an even more ridiculous amount of health regen as well. There is nothing to say about this except GET IT!

Warmogs Armor
This is argueable. I like this item because of the ******ed amount of hp it adds, and a great amount of health regen to stack with Force of Nature. The added hp from this also increases Force of Natures hp regen. If you feel that you don't need the extra hp, and would prefer more survivability, then go for something like guardians angel.