I play this build primary against Ad teams, because i find it very useful and fun to play with, it's good for almost every situation in Summoners Rift. Regardless witch Ad team you are against.

I choose Greater Quintessence of Fortitude and Greater Mark of Fortitude because one of Rammus' great forces is hes massive health and also because we are against a Ad team witch is why it would be good with these runes if you are playing against a Tryndamere and u have thornmail so you just have to hold you health up to let him kill himself. I choose Greater Glyph of Warding because you do almost not fight against a complete Ad team so it's always good to have some resistance for magic anyway. And finally I choose Greater Seal of Resilience because you are against such a physical enemy so you will need armor in this battle to keep you alive and be the tank of the team.

I've picked these masteries primary for playing against Ad teams, and when you do that you will, as i said earlier, need alot of health and armor. Why i have those points in masteries is because when you are tank, you want to spend less time dead because the team can't fight without you. I found out that when laning Rammus empties hes mana pool very fast so Expanded Mind may help you a little. and then Swiftness because it's good for almost every situation, when u have to flee using Powerball and engage fights with some more speed than the enemy so you can catch them up and use a taunt and then pull them back to your team so they can pick up the kill(s).

These items are for a 5v5 Summorners Rift nothin else. When that's said you primary want alot of armor health and some magic resistance at last because almost no team is clear Ad. The first thing I build is Mercury's Treads it'll give you some early game Tenacity and a little magic resistance. Frozen Heart is a very good item for Rammus and i build it very early, mostly because of hes lacking mana pool and big cooldowns, and as a bonus it gives alot of early game armor. Force Of Nature as I've said many times before, no team has clear Ad and then it'll provide a nice amount of health regeneration. Warmogs Armor because of health, you can't stand a whole game with no health so depending on how feeded the other team is, I build it 4th. But it could be earlier. Thornmail is a very useful item too, mostly because of its ability to return 30% of the damage taken as magic damage, again it depends on how feeded the other team is, when i build it. At last Randuin's Omen one of my favourite items, it has everything Rammus needs and a little more, the reason why i don't build it early game is because of lack in Rammus' early game power as a tank Randuin's Omen provides a little of it all but you'll need to focus armor, mana and health in the beginning.

I pick Heal as one of my two Summoner Spells because when i play with my ranked team i usually play Rammus and out support has Clarity and Clairvoyance so I would have Heal, not only to safe myself but to use it in team fight for the whole team. If needed. And then of course Flash it's always useful to make an escape or catch up with an enemy even when you are Rammus and have Powerball.

I made this guide because I've alot of experience plating Rammus and hes a very OP champion if you use him well. when I engage a fight I can come from a bush where the enemy haven't seen me, then I come out with Powerball, target the most feeded enemy, if posible, Taunt him, use Defensive Ball Curl, perhaps Tremors if hes teammates keep up to me and try to pull him back towards my team so they can pick up the kill.