This is my most recent rammus build. I was tired of just mainly being a tank and wanted to contribute by truely dominating the enemy. My main focus with the build is trying to gain as much health and ability power through out the game. The runes will allow great early game pressure. The enemy will do very little damage and since the passive is 25% of armor is converted into AD, then each base attack is about 75.
NOTE* this build should change when vs specific characters. Such as a tryn or a yi, in which case a thornmail is key to victory. Also if there is early game magic pressure, get the magic resist up fast.

Note*- the following is copied from Bgugg's guide. Credits to him for writing the following, i couldnt do a more detailed job in explaining the ability portion of this guide.

Powerball
Rammus accelerates in a ball towards his enemies, dealing 100/150/200/250/300 (+1) magic damage and causing 28/36/44/52/60% reduced movement speed to nearby enemies on impact.
Cooldown - 10/10/10/10/10 seconds
Cost - 80/90/100/110/120 Mana
Range - 300

** Hits all enemies in a 300 AoE.**
**Right clicking or A-clicking an enemy will not cause you to ram into them. Instead, you will stop just short of their model, in the position to autoattack. To ensure a hit, click a spot next to or behind them.**
** In order to hit Golem, Dragon, or a big Cho'Gath, you must collide with the back of their hitbox.**
** Powerball continues through Teleport: this can surprise enemies (props to Jemrin for the reminder).**
** Can be canceled at any time by pressing Q again.**

Defensive Ball Curl
Rammus goes into a defensive formation for 6 seconds, increasing his armor and magic resistance by 50/75/100/125/150 and returning 26/32/38/44/50[(+0.10)*(armor)] magic damage to attackers.
Cooldown - 14/14/14/14/14 seconds
Cost - 50/50/50/50/50 Mana
Range - 300

**You cannot use Powerball with Defensive Ball Curl(DBC) on.**
**Stops motion for .3(ish) seconds on cast.**
**Can be canceled at any time by tapping W again.**
**Damage return procs spell vamp (courtesy Civil Enemy).**
**Does not proc on Sivir's Ricochet or incoming Thornmail damage (courtesy Civil Enemy).**


Puncturing Taunt
Rammus taunts the unit into a reckless assault, reducing their armor by 10/15/20/25/30 and forcing them to attack Rammus for 1/1.5/2/2.5/3 second(s).
Cooldown - 9/9/9/9/9 seconds
Cost - 50/60/70/80/90 Mana
Range - 325

** Sometimes bugs a bit if at maximum 325 range. To ensure target switch get closer to enemy.**
** Remember enemy doesn't hit you immediately when you taunt. There is a brief time before they attack as they switch targets and move over to you or their attack cooldown timer ticks (this is important for tower aggro).**
** Cleansable.**
**Switches target of all minions except Baron.**


Tremors
Rammus creates tremors beneath him, dealing 65/130/195 (+0.3) magic damage per second to nearby units and structures.
Lasts 8 seconds.
Cooldown - 60 seconds
Cost - 120/120/120 Mana
Range - 300

**The damage is entirely independent of the animation, it will strike once per second (regardless of the apparent pulses) and a little bit beyond the range of the animation.**
**Continues during Powerball use.**
**The AoE hits through thin walls.**

Focus on all armor. From then on, armor really isnt as important in the game. Rammus should be set. He will take little damage to minions or even champions and will do a lot of early game damage.

Rammus is a tank so you should build him as such. Get as much resist as you possibly can and then go for some ability power and ability cooldowns.

It is always important to get the rod of ages first. That will give you plenty of mana so that you can continiualy roll around and inflict pain on the enemy. It will also greatly increase your health. Warmog will also increase your health as will the scepter. With your build, focus on as much health as possible while also considering the ability power. You will already resist so much damage that there is no need to continue to increase it so therefore, it is time to inflict some of that damage abck at the enemy.

Flash is key for getting out of sticky situations and it is also perfect for pouncing on the enemy while rolling in.

Ignite will come in handy when fighting the enemy champions. Just ignite the enemy before or after engaging. I prefer before because the enemy tends to never be able to escape. And while doing so before, you inflict a lot of damage on the opponent.

Rammus should initiate into the battles. Always! With his roll, go full speed directly at the strongest enemy champion. The apply taunt followed by the defensive curl and lastly the tremors. Since you will have so much health, you should be staying alive this entire time while dealing an insane amount of damage. These tremeors, when fully built, will do over 1 thousand damage over a 8 second span. The enemy will go abck into a full retreat. With the scepter, all the units are slowed int he process of running away as well. Now, team work is key. You cant expect to run in there and get a penta kill. You must work together focusing down the strongest enemy champion. Also, be careful of stuns resulting in the enemy etam nuking you. However, even though you will get massive damage done to yourself, you still did your job as a tank to absorb all the damage in. You team, hopefuly will be somewhat decent, to counter the enemy and come finish them off.

This build will allow rammus to truly be an offensive threat as well as a tank for your team. I highly recomend all tank players to try this build out. The enemy wont be able to run away. They wont be able to kill you. Rammus will be a frightening foe that will inflict pain into the enemy.