PROS:
strong late game
good ganking ability
high damage output
good sustain
great for dueling
very fun

CONS:
hard early game
susceptible to cc
weak to magic damage

Hello guys! This is my first build that I've ever posted here. Me and my friends came up with this build by messing around with my favourite champ, Rammus. This build can give your Rammus play-style some pizzaz. It will send your enemies running and have your allies praising you. It will also give you tankiness for early game, sustainability mid game, and high damage output late game.

I'll be updating this guide again... hopefully soon, but I hope it helps you guys out.

The masteries I've chosen should generally give you enough tankiness, and some decent damage for last hitting to survive the early game. Generally you would be playing Rammus in the top lane for this build(I'll upload a jungle ad rammus guide in the future).

While you're in top lane you need to be able to stand hard engages which is why decided to max out Defensive Ball Curl first. Rammus' W will give you a high amount of physical and magical defense plus whenever they try to damage you they'll be hurting themselves too.

I usually take his Puncturing Taunt at level 2 just in case my lane opponent makes the mistake of pushing close to my turret. You'll generally be facing melee champions at top so this is an ideal ability for when they're underneath your turret trying to push. Combo this with your W and your enemy will be taking damage from the turret, your W, you basic attacks, and any nearby allied minion.(Your opponent will usually be too scared to push your tower after the first time you've done this.)

Rammus' Powerball is a one-level-wonder. Meaning that it gets the job done at its first level. This is generally taken at level 3 to escape from early ganks. Using this can also be used after you've taunted an opponent under your turret to stop them from escaping the wrath of your tower.

I go for the general start of Mobility Boots and 3 health pots for some sustainability in lane. After your first back to base you should get a Cloth Armor or Chain Vest which will build into a Thornmail later on. You should follow this up with Ninja Tabi. Ninja tabi reduces 10% of all basic attack damage and it supplies you with some more armor. Avarice Blade is another good item that can help you farm in lane and give you some extra gold/5seconds.(Avarice blade might not be needed later. It's just needed to accumulate more gold, so it should be sold late to make room for another item.)

If you find yourself needing more sustainability in lane get a Vampiric Scepter. Otherwise you should rush a B.F. sword for extra damage. These will build into a Bloodthirster which will greatly increase your dueling(1v1) abilities.

The next item you will need is a Trinity Force. During a fight you will be casting your abilities a lot which gives you a lot of opportunities to proc and apply trinity force's Sheen(making your next basic attack do triple damage)

Your final two items are to help with your critical damage. The Infinity Edge and Phantom Dancer will increase your critical chance by 55% together, and inifinity edge's passive increases the damage your critical strikes do to 2.5 times.

HERE'S SOME MATH YOU DON'T HAVE TO READ IT:
Infinity Edge + Phantom Dancer + Trinity Force = 65% critical chance. If you include the passive of Infinity Edge that means if you attack 100 times in a row with 325 AD and 65 of them crit, you will deal [325AD * 2.5 * 65 + (325AD * 35)] = 64,187.5 physical damage(not taking account opponent's armor)

Thornmail, Ninja Tabi, and Bloodthirster will let you win any 1v1 engage. They are you core items. Thornmail's damage will stack with your Defensive Ball Curl damage making your taunt a force to be wreckened with. Ninja Tabi will negate 10% of basic attack damage. These two items will increase your damage output and decrease enemy damage output while taunting or dueling an ADC. Bloodthirster will give you lifesteal so that you can recover any health that you've lost during a fight.

Mercury Treads is a good replacement for Ninja Tabi if you are going against a team with high magic damage, and the tenacity is nice for heavy cc.

Ravenous Hydra and Iceborn Gauntlet can replace Bloodthirster or Trinity Force respectively if you find yourself in more teamfights than usual and you need some extra tankiness and AoE damage.

Atma's Impaler can be a viable item since it gives some armor, AD according to how much health you have, and a nice critical strike chance.

Black Cleaver will help you against those tankier opponents who are stacking armor to negate some of your damage. Reduce their armor and make them feel sorry for buying it.

Rammus is an unstoppable bruiser that can destroy the squishy enemy carries with ease. During the late game 1v1s are your speciality, so be confident in your engages.

Sorry the guide is so short and incomplete right now. I was pretty excited to post it. I'll be updating it regularly to make sure its the guide you guys deserve.