i will tell you why.

1. Great defence, rammys converts 25% of his armour into atack damage, witch means you dont have to buy damage items to kill enemis.

2. at late games (if you have completed this build) you are unstopable. turrent does not hit you alot. so you can easy tank turrents. (remeber to use defensive ball curl spell)

3. since i chose boots of swiftnes, and flash i can realy realy easy escape from enemis. if you combine this with powerball no champs can catch up with you. this means rammus can be the fastest champ.

There is only one reason for this. i realy hate rammus in early game, cuz his hp is realy realy bad.
but if you want, you can use some armour runes instead. thats your choise.

Rammus is defently my favourite tank. easy escape metods, and great tank abilities.

Champion Spells:

Spiked Shell

Rammus gains additional damage as his shell becomes reinforced, converting 25% of his armor into Attack Damage.
[/color]

Powerball

Rammus accelerates in a ball towards his enemies, dealing damage and slowing targets affected by the impact.

Defensive Ball Curl

Rammus goes into a defensive formation, vastly increasing his Armor and Magic Resist, while returning damage to attacks.

Puncturing Taunt

Rammus taunts an enemy unit into a reckless assault against Rammus' hard shell, reducing Armor temporarily.

Tremors

Rammus creates waves of destruction pulsing through the ground, causing damage to units and structures near him.

Source (http://leaguecraft.com/champion/Rammus)

Rammus is a realy good assist champ, but also a good killstealer in teamfight. just turn on your ultimate, and you will atleast get some of the kills. bu dont expect to get alot of kills. but you will get lots of assists. as you can se at these pictures.

If you have questions about this guide, i would love to answer.

This guide is not finished yet, and will be more updated soon.

I also need to apologise for my bad English.

If you like or dislike this guide plz comment.