Not Updated For Current Season
Okay, disclaimer: I am not level 30, so the Mastery tree is just what I am going to do, and I haven't done the runes, you can do those based on what YOU feel you want.
Also, this build is for a tank, not a killer. Expect a lot of assists, not kills (unless you can pull someone to your turret with your taunt, or happen to catch a squishy alone).
Alright, so I've been playing Rammus for a little while, and haven't found a truly satisfactory build for me. I found a Shen build by Terroxfury and made a slight hybrid of his, and with it, I have found that my survivability and and utility in teamfights is outrageous.
Spells: I go with heal and Ghost for easiness to get around and survivability, you can go with ignite if you are intent on trying to get more kills, or smite if you want to jungle a little, both are viable.
Start with Boots of Speed and 3 Health Pots
Ninja Tabi: Armor (and attack because of Rammus' Passive) boost. The dodge doesn't hurt either
Sunfire Cape: Health/Armor and the passive in combination with your ult makes you great in team battles, just know that when you walk by monsters, you aggro, and you can't hide in brush, because the damage aggros minions.
Banshee's Veil: Health/Mana/Magic Resist and deflect one harmful spell. Very helpful when fighting AP heroes.
Warmog's Armor: The insane health boost is perfect for a tank.
Guardian Angel: I have yet to make this in a fight, but it's usefulness is that it keeps you in fights just a little longer, and can possible get you out of a death if combined with ghost and powerball.
Thornmail is a good alternative to almost any item in the build, feel free to use it.
If you have a quick team you can score an early gank (usually what I do) by taking 3 characters to a lane, and hiding in the brush, let them ambush then you rush in with your taunt. Eureka, easy first blood. After this, return to your lane or send your 3rd on their way. Essentially, just focus on last hitting minions and maybe catch a dummy being brave and taunt him into your turret. Hopefully before too long you can get your Ninja Tabi.
You still aren't quite ready to lead the team in team fights, so if one occurs, power ball in, use your shield, pop your ult, and Ghost out.
Soon as you grab Banshee's veil, use your ult/shield/heal combo and you are a damage machine getting assists left and right. Not to mention that your ult hurts turrets, so even champions who are turret humping do not stand a chance, and it is often a good idea to instigate a fight under their turrets.
Alright, I know that isn't too in-depth, but as I learn Rammus more, I will update, Feel free to comment and share how the build works for you. Farewell, and have fun.