Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Rammus' ability to gank is incredible and if you ask me, highly under-used, initiating with powerball from brush then following up with his E and W skills make a gank incredibly easy especially if the target is a physical damage champion.
This is more of an off-tank build of Rammus, with reasonably low health compared to other tanks but a high attack due to his passive, the build focuses mainly on taunting high physical damage champions such as Miss Fortune, Ashe or Caitlyn in combination with his second skill defensive ball curl. I use Rammus' powerball ability simply to engage with as the splash and slow are incredibly good for initiating and chasing down opponents.
With the build that I use, it is very good in team fights to try and taunt the high physical damage champions, this is very easy on champions such as Xin Zhao or Master Yi. Rammus' E in combination with his W skill make it very easy to drop champions like Xin or Yi EXTREMELY quickly, in my experience you want to try to initiate with the powerball ability for best effect, the slow and splash damage created make most enemy teams want to run away and then spam your E skill on any high physical damage champions OR if one of their caster's is getting a little too powerful it is always good to silence them with a taunt, this in combination with his W skill is a dangerous force, during team fights you will always want to try and have his W skill active.
Creeping / Jungling
During early game the two buffs from golem and lizard can be very useful for Rammus, with the build I use, you will often find yourself running low on mana if harrasing a lot so the mana regen can be amazing, and the red buff is good on any tanking champion imo.