Note that some of the parts in your guide contains flaws

Marks are best for offensive based runes, meaning MrPen as well as Armor Pen, attack speed etc. SS doesn't really stand for missing, and even if it really is and I don't know, it isn't really common, MIA and btm g etc is better. Perseverance is useless seeing your regen sucks (Late game you don't really care much about regen, so when its high it probably doesn't matter) whilist good hands can take off about 7 seconds at level 18. Higher Max MP by 5% is USELESS on Rammus. If a champion with 3k MP takes the 5% boost, he gets 3k X 105% which makes 3150 MP. Rammus doesn't even have 1.5k, so he gets roughly 67.5 extra mana (1350 X 5%), which is totally ****, exp gain is better. Meditation can replace greed, but greed actually is OK on supports and tanks seeing their farm SHOULD BE bad, be it good opponents or giving it to the carries/spellcasters. The items also should be reordered to defend against a different team, this is a pure anti-AD build, replacing any one of the items for Force of Nature or Banshee's Veil would be a MUCH better choice, as well as against a balanced team, warmog should be gotten 1st. The best choice to be replaced for a Mr Item is frozen heart, you don't need the cdr, the ton of armor keeps becoming less % damage reduction every 20 armor (give or take) and Randuin's Omen ALREADY has the slow attack speed ability. Guardian Angel should be gotten LAST or close to it, there is no point for a semi-glass cannon to take it when they easily die, might as well take much tankier items before you get its awesome ability.

Hope it helps :)

Nice guide...I like to use flash on rammus because you can flash as you powerball so you can use it to flash over minions to hit someone in a fight :)

I would also incorporate some items versus AP teams. You should make a seprate section for it :)

Other than that thumbs up