Most builds for Rammus explain him to be a tank... Purely a tank...
But he can also be played in an aggressive manner...

Spiked Shell: The bonus from this ability dose not seem to be calculated... This build would add 113 additional damage, resulting in a whooping, yes my friends, whooping, 245 damage... Might not seem to be a lot, but for a tanking hero it's quite good...

Powerball: to charge in fast and chase down heroes...
Defensive Ball Curl: only later on in the game when people will have high damage output...
Puncturing Taunt: to turn back heroes that seem to want to run...
Tremors: when fighting multiple heroes...

Summoner spells:
Exhaust: single target slow. Quite good for chasing down heroes... The talent in Offense for the 0.5 seconds to duration, which if a lot in a fight... It still is a Puncturing Taunt thrown in 0.5 seconds later...
Ghost: same reason as Powerball... The Talent in Utility for the slight speed bonus and the quite impressive 1.5 seconds bonus duration... It dose count when you are chasing a hero...

Items:
Randuin's Omen: AoE slow. Same reason as Exhaust...

This build permits him to easily chase down heroes, while still pushing the line...

The talents in Defensive are purely for tanking purpose...
Strength of Spirit: This talent is not so important for Rammus, because it only gives him a 8.5 HP per 5 sec regeneration... If you grab more mana items it permits you to jungle fast between hero fights...

The talents in Utility are for a faster progression...
Good Hands: Les time dead means more time fighting... And 10% might not seem much, but in late game when you spend one minute doing literally nothing it dose count...
Awareness: The bonus 5% experience might not seem to substantial, but it\\at one point you will be one, maybe two levels over your enemy level...