The way I see it, for Rammus defense is his top priority, therefor everything should work towards his defense. However, he is slow, he has good useful abilities and little mana, and one of his biggest advantages is the thorns effect that he has. I mainly wanted to try and make an all round build that keeps his defense immensely high and therefore his damage is good too.

The items are the most important part. If you have the option to increase his defense to such an amount that his attack damage is at it may have been if you had bought any ad item then I think it would be better just to get the defense, and you still have to consider all the above factors.
My first choice is Ninja Tabi, these are good because you get much needed movement speed, a little damage reduction, (which doesn't effect his defense curl because no matter what damage is dealt to him it will always give the same magic damage) and 25 armor which gives 5 ad.

Thornmail is also essential. 100 defense and even more damage returned to your attacker, which the boots do effect this but not by too much an ammount.

Randuin's Omen is the next peice. It gives a bit of much needed health, 75 more armor, some health regen, a little cooldown reduction, and now those pesky enemy champions who are faster than you have a chance to slow down a bit with the nice activated ability provided by the item. All round a must have.

Zonya's Hourglass to me is another good addition to have in your stash. It gives a huge 100 ap which nicely beefs up your abilities and 50 more defense. In addition to this you get a neat ability to be invulnerable for a short time if need be, and being a tank may be useful since your allies will probably be behind you most the time. (On a side note, I am not sure if being invulnerable lets damage be returned)

Ok, so our defense is working nicely we have alright movement and a good amount of ap for how he works, what about his attack speed, he is still moving slow. The Phantom Dancer is always a good choice, now he has excellent attack speed and movement plus he can get those nice crits from time to time.

So now your a beast that can crush through physical damage like a car through flies but those pesky wizards and ap champions are still quite the thorn in your side. The Guardian Angel has a nice bit of armor (as if we need more) a good amount of magic resistance, and if they still manage to stop you, they gotta get you twice now on top of all that.

As for the mana problem early game, just get some mana pots and late game it usually evens out nice for me.

I'd say this makes one mean Rammus.

In how I pick my abilities, I give the most weight to his powerball. It is the ability I use the most and it works in many situations.
It is great for charging in out of no where to surprise some enemy champions. I also use it a lot to help friends escape by rushing in to slow enemy pursuers down. Also, it makes a great way of running away if need be. You can even use it to simply move faster to get somewhere quick. Over all, it is the most useful.

Although I do put my emphasis on powerball the defense curl ability also is extremely important and I like to keep that one either one level behind or on the same level as powerball. It increases armor, magic resist, and on top of that has recoil magic damage to attacking foes.

His taunt ability is amazing, however, I never really need to slow someone down more than a little bit to get an advantage over them, so I always fill this one in last. Also, I don't get much chances to really utilize this till late game.

His ultimate, Tremors, I will usually save it for battles against champions, the extra damage on top of the recoil damage and the attack damage helps a lot. But it's extremely low cooldown rate for how useful I think it is makes it a great tool for farming or helping destroy towers, because not too long after its ready again.

So Rammus gets Beefy.

I didn't go into masteries or runes because I think they are extremely self explanatory and I'm tired and really don't like typing a lot. They basically emphasize defense, damage reduction, and movement speed.