Rammus in my opinion is the best tank, my favorite to play by far.
His powerball speeds him up pretty well, and does damage and slows when you make contact. My favorite thing to do is have flash so that you can activate your power ball behind a wall of minions, and your enemies wont think a thing of it. Thats when you flash past the minions and hit them. always fun =D
His shell ability increases his armor by a ton (at max level increases armor by 100) and returns 20% damage done to him in magic damage
His taunt is amazing, it taunts people for a couple of seconds allowing your team to pounce on them and reduces their armor significantly
His ult is great as well, he shakes the ground around him and deals some pretty decent damage in waves for a few seconds, its an amazing DPS and very useful in a team fight. He is able to move around while doing this, it also damages towers and inhibitors.
His passive is awesome for a tank, it converts 25% (i think) of his armor into attack damage. He is by far my favorite tank to play, i have the most fun with him