Not Updated For Current Season
This is a build I use with Rammus, late game I simply can not die and can take 3v1 without going below half health, especially if I'm going against a melee oriented team.
Items wise, you can basically get whatever you want after the first sunfire cape, it doesn't really matter, by then you'll be able to take anybody down without a problem. But I suggest you get an Aegis of the Legion and then either stack Thornmail or Sunfire capes, or a bit of both.
Scenario: You are in a bush, the 3 people on the other team are in a lane, pushing, this situation looks bad, yes? Correction, no, it doesn't, keep reading. You powerball in, making sure to charge it for about 5 seconds in the bush so they cant run, and then put a taunt up on the person you plan on killing first,and then use Tremors as soon as you can, followed by defense ball curl, at this moment, people should still think they can kill you, but will soon realize your thornmail and sunfire capes mixed with your skills has bested them, and they will start to run. What I try to do when I know some is about to run while I'm using tremors, is start to run before them, make sure I'm about one champion length in front of them, so the tremors still hit them, and if i can taunt them or powerball them away from where they are trying to go, this works almost all of the time. Be careful though, Rammus can be squishy without items, don't be over confident in the beginning of a match.
At the end of the match, if you have gotten all of your items, you should have quite a bit of armor, and even more armor when you use defensive ball curl, and you will still deal a lot of damage, seeing as 25% of your armor is turned into damage, which I find pretty sweet.
The summoner spells can be changed up, but these two are just what I use most.
Sorry for the unprofessionalism of this guide, but I hope it helps, as it has helped me much. It can be used for 3v3 or 5v5.
PS Comments + Criticism greatly appreciated.