So I start with the boots cause Rammus is ridiculously slow and 3 health pots goes a long way when laning. This is my build that I have used for 1v2 when my teammate is jungling and I can usually get a good amount of kills and maybe even first blood because Rammus + Turret + Aggressive enemy = Dead Enemy.

OK so i know there are 7(core) items, the first six were my old build and depending on what the other team had determined whether I got thornmail early or Force of Nature. Now with Zonyas hourglass I've been messing around and I think its better to not get Thornmail/FON depending on what the other teams Champions are. Same goes for boots switch to Mercury treads of the other team has a high amount of casters. If i get superfed fast, and turn into carry rammus then sometimes ill sell the boots and get whatever item I dont have before. Even though you'll be super slow with 3 elixirs and your powerball/Ghost you can escape a lot. Sometimes I take away Masteries from defense so I can get the buff duration increase because with both buffs and elixirs Rammus just wont die.

Now Randuins Omen and Frozen Heart are both great items to get. If the other team is stacked on physical dps and Champions who will be hitting you alot with their normal attack grab those 2 items and stack armor so that your Defensive Ball Curl and thormail will decimate them.

Sunfire cape is opptional and probably the item you can get rid of to get situational items. I just prefer it because combined with Defensive Ball Curl you can farm in seconds and with taunt you cant deny free dps.