Soooooo this guide will be a guide for rammus being a good tank in 3v3 mainly but 5v5 will work even if hes not jungling

I take nine armor penetration runes, for that extra damage early game, along with two quintessences for armor penetration for the same reason, one critical quintessence because nearly 5% crit in the beginning is a real nice one for first blood, for glyphs, i take nine magic penetration for those AP champs that are a pain to kill, easier to kill, one critical damage seals to make it 5% critical damage eight armour runes to make rammus more tanky, at the same time, more attack power

I put up flash and ignite because, with powerball, flash is even more useful, since at the beginning, powerball isnt that fast and you might not get away but with flash, youll move further and have that extra movement speed, ignite to kill that enemy whos about to die, die

In team fights, especially late game, rammus is quite the tank, you wanna run in with powerball, and if your tanky enough take on a few at a time, slowing them all then focus one of them with your taunt followed by your shield and tremors, as the rest of your team should be doing the rest. the good thing about rammus's taunt is that even when someone trys to run away, you can still get them by your powerball + taunt. Slowing them and taunting while you attack them could get some of your team members to come up and go for the kill.

Pros
-Super Tanky late game
-Quite op if guide is followed
-Passive allows him to do quite alot of damage even w/o a build for them
-ult gets nearly double from upgrading it twice
Cons
-Not the best early game
-Hard to master

I take regrowth pendant and a health potion first because it gives you that extra boost of health regenoration at the beginning in case you get too hurt, then onto philosophers stone for the mana regen and more money :D. Next i get boots of speed, the reason i dont get treads till the last item is because rammus's powerball is more then enough movement speed, plus youll have more money for heart of gold, to give you more health that you'll need. Next up i go for a sunfire cape so when you taunt people, they'll be hurt from the cape plus your attacks, also that armour bonus resulting in attack power. After that i rush a guardians angel, so you wont have the fear of dieing plus more armour. thornmail is a good choice after because of that 100+ armour and ememies will also get hurt if they try attacking you. Omens will give that that extra armour youll need for more attack power also health regen.

If your lucky, rammus wont get banned in ranked games, and you can own with him :D