Please be aware, Rammas is a tank this guide is just a general idea of how I build him. It is my core build. I deviated from it as needed depending on the other team.

I play an aggressive backdoor Rammus with the option of rolling in to assist a lane with a gank. If I have a glacial Shroud, Thornmail and Ninja Tabi at lvl 11; I love to roll up to a turret, drop into my def. curl and hit my ulti. When my def. curl runs out I can pop a heal and finish a turret off with autoattack, or roll away (if enemy champs are teaming up on me). With 15% CDR I am not afraid of "wasting" my ulti to clear a large minion wave. It is, in my opinion, the only effective way to farm with Rammus.

I look at runes as a way to boost my early game. Because of this I have selected runes that can give me a slight edge on lvls 1-5.

Armor Pen helps auto attack, Magic Pen helps def. curl and ulti. Dodge runes are very useful. My philosophy is, if I'm not returning damage from def. curl, I'm dodging it. I also have a magic resist and AP runes. (ulti stacks with AP)

My core build is always Ninja Tabi then Thornmail. Rammus's def curl and passive stack with armor making Thornmail a requirement (even against an AP team)! I prefer Glacial Shroud and late game upgrade to Frozen Heart over Aegis of legion. The CDR and mana boost help with spamming def. curl. Purchasing a blue stone is enough to avoid mana problems. Sunfire Cape allows Rammas to farm and inflicts damage while the enemy is taunted. Without it you will be lucky to get 50 minion kills. Depending on the strength of the AP champs on the other team and who I am laning against determines when or if I get Force of Nature (Rammas is weak to AP). If its an all AD team I often purchase a second Thornmail. The passive doesn't stack but the 100 armor does.

I always start with W, then E, then Q. Level 1 Q is enough to help you move around the map. After that I max W, E, and R in whatever order the game play demands. (Do I need a longer taunt because I'm ganking or more def. curl because I'm tanking turrets?) I often find its more effective to keep taunt 1 level above def. curl l.

I play with Heal and Ignite like a noob. I consider flash if only used for a spped boost to be wasted slot because of the power ball. I have enough taunt/slows from skills and items that exhaust seems redundant. Chances are if they are escaping me they don't have much health and are running into turret range. I'd rather hit them with ignite and stand back, then chase them and die to the turret.