Rammus is pretty much the best and most solid tank out there hands down. Although able to play from various angles, this build seems to work the best as it focuses on returning attacker's damage to them, being a strongly standard and basic rammus build. Heavily built on armor to make use of Defensive ball curl, and his passive (Converts 25% of armor into attack power)for tanking, and soloing towers if all else fails, this build ss primarily used for use against teams that have 3 physical damage and 2 magic, although slight tweeks can really make rammus tankable in any situation. Rammus is a champion that, as a tank, must be built around the enemy team. This will be discussed in the Items section, so jump there if you'd like. Otherwise, Follow this very sturdy and standard Rammus build which will make Enemy's regret attacking you, as they will eventually end up doing more damage to themselves than to you.
Some of my scores:

The runes here are built for simple reason. This build attempts to eliminate damage taken from all sources for simple tanking. Therefore; magic resist runes are perfect as it helps highly reduce the magic damage taken until a banshee's veil can be purchased. This allows a banshee's veil to be pushed back in the build order so more important things can be acquired such as thornmail and ninja tabi for the purpose of this build. The Dodge Runes simply add to boost ninja tabi's effect, allowing for more dodge, another way to take less damage.

Oh but it is. Pretty common sense on the defensive part of the build, 2 points spend on reducing monster and minion damage, this is again for early game and it helps for tanking/or/soloing dragon as soon as possible. Improved heal and fortify are used since your using the spell, especially since improved fortify is such a big improvement. (adds 50% splash damage for quick minion kills) the points added in health/mana regen are for early game, as rammus wants to focus on last hitting and laning as long as possible since tank item costs are so ridiculously high. The point's spent in AP on both defensive and offensive tree's are spent to help give a boost to tremors and powerball, however slight it may be, this still happens to be the best choices.

Again, Rammus is a tank, so he must be built around the enemy team, ALTHOUGH, he still does need to stack armor for defensive ball curl and his passive to be effective. Above lays the standard build, but rearrangement of items and extreme potion consumption may be needed if you happen to be facing a team of; karthas lux ezreal annie and Galio <- yes this has happened to me before.

Many rammus builds advise using a Force of Nature somewhere in their build. health regen and magic resist are high and it is useful yet expensive. Force of nature does nothing to add on to this build as A. it does not add to turret pushing power as it does not prevent turret damage taken, nor does it make use of his passive to strengthen his attack. B. Banshee's veil is a better item due to it's passive which absorbs a negative enemy spell every thirty seconds. simple math concludes that completely removing a spell from the burst damage taken at the beginning of a team fight (which at this point might be led by say, ryze or heimerdinger as the carry with immense range to get first hit on you) will net less damage taken overall then with the 76 magic resist magic reduction given by a force of nature. This is only due to the fact that stacking magic resist runes, gives -61% magic reduction at the end of this expensive build. with Defensive Ball Curl added to that, magic damage reduction will end at say 75% roughly. the extra 50 magic resist will only boost magic damage reduction to roughly 79-81% damage reduction at its peak. the 4% extra damage taken at this point throughout a whole fight will not amount to as much as ignoring an entire Hextech Micro-Rockets. This equation will be at an extreme end game and based on teams having even kills, AND a A.P. building heimerdinger using my custom build (will be on mobafire soon) with around 725. Damage taken by the rockets would be a canceled out 163 damage due to banshee's. (this is the number that comes from the total damage (683.75 - 525 due to magic resist). On the other hand the damage then taken when using FoN would be 147.
But banshee's works once per 30 seconds, so you would then continue to take an extra 4% damage from all magic sources, which, unless fighting against a teemo, tend to have high cooldowns. Now 4% extra damage from 6-10 sources (various abilites, say a lifedrain by fiddle, etc) will not be more then the damage possibly taken by the rockets as most heimer builds include at least one source of magic resist penetration or reduction. This then means that the inital damage would have been greater, and while true that that increases chances of force of nature having higher damage reduction, it must also be taken into account the cooldowns and team fight and solo senario. The math gets really down to the nitty gritty here, but suffice to say that the burst damage prevented from mage carries with moderately high cooldowns will provide the tank with enough health to charge, taunt, and let his carries/dps to kill enemy carries while they are on cooldown as the majority of their damage was then ignored. Here banshee's does have a flaw as in any spell can take the effect away, even Zilean's Time Warp. this still helps taunt and catch enemy carries or, removes a stun allowing for defensive ball curl to be cast (removing early CC)) which would have cause MASSIVE (in the range of 15% extra damage from magic sources) instead of one constant stun being placed on tank while all carries are destroyed, or until tank dies, in which case an ace situation occurs.

If the enemy team is Primarily a magic using team, it is recommended to rush banshees veil to the front after Heart of Gold, and then throw a Force of Nature right after. then resume building thornmail, and ninja tabi. After you would really have to follow your gut, if still being heckled by this team, then get guardians angel right away, if not, then build sunfire and warmogs. If further being beasted after guardian's angel, and you somehow didn't get wrecked enough to lose but were able to afford all of this armor, it might be best to sell your magic resist items, and just follow the origional build, and try backdooring as rammus as he is a very effective backdoor/turret solo'er. If you refuse to do that then just continue building magic resist items until you can effectively tank, although once guardians angel is acquired you should be set.

Heal is the best summoner spell for a tank. Hands down, when coupled with the improvement mastery, heal if a very effective spell when laning with an ally, and when tanking huge team fights a huge team heal can make a difference between being aced, and acing a team.
Fortify is a support spell that i prefer when playing rammus as the improvement mastery is readily available and helps stop pushes if an ace happens or If a cocky jungling warwick and an evelyn happen to gank a lane and attempt to push the turrets quickly. casting fortify then does splash damage to any minions led and the champions, quickly dispelling the effort until allies can rush to the turrets protection. The only real con to this spell is the long cooldown time. There is no way around it and at times you may wish you would have chosen another spell, but a good team will not need to worry about turrets being pushed too much anyway, so chances are you'll have the cooldown spent by the time your turret is in danger.
Flash is, in my opinion due to my playstyle, a horrible rammus spell. I don't even use it, but to be fair, i don't use it on anyone. I find the limitless reasons to use it matched with limitless reasons to use other spells instead. Riot is rumored to be removing the spell anyway. Power-ball contains all the running and chasing power you need. A popular combo is flash balling enemy's in the face though, using a tactic said below. Although as stated by a commenter below, a jungling rammus paired with flash and smite may jungle, flash over a wall, and power ball one enemy into another causing for an easy gank. these spells though, are not tanking spells, as smite becomes near useless once the laning phase ends, and flash is used as an escape / gank mechanism, which, although useful with a taunt combination, is superceded by the total amount of deaths possibly prevented using heal, or the free push prevented by fortify. Here some would say that ignite could be used over smite for total game usefulness, and again, this is true, its a good combo and for many playstyles this will work, but chances are the other 3/5ths of your team will be using it and clairvoyance, so you have no need to use them, especially in team fight situations.
Ghost is slightly acceptable as power-ball does cost a bit of mana, but honestly, heal and fortify are much better choices for the team help factor.
Clarity can be used if laning with another heavily mana Dependant person such as Annie although it might just be best if they use it.
Ignite + exhause just makes you look like a desperate rammus honestly. While it is true rammus lacks the potency to catch the final hits on some enemies, and this does help laning early on, rammus is a tank, so expect to help your laning partner get these assists, or try to power-ball the enemy backward into your allies. These are still acceptable though.
Clairvoyance Simply awesome, yet better for another player to use.
Rally DON'T Just...Just Don't.
Revive What are you level 1? using revive is a signal to everyone that your expecting to die. Mainly because revive is a spell mainly used by people so tired of dying. true it is useful when spawn times are 90 seconds, but honestly, you want to win before spawntimes get that long.
Cleanse Good spell, especially now with all the CC champions out. good for tanking although, CC doesn't really matter as you can take all the stuns and the such because you have the health and damage reduction for your rammus to be half health + by the time all the carries' cooldowns are up.
Smite jungle rammus is aweful. end of story. This is also not a jungle rammus build, dont even touch.
Teleport Great spell, not for rammus. could be used for ganking, easy defence, pretty much anything, can even easily switch masteries to get improved teleport, but Heal and fortify are great team combo's. if you refuse to use either of those spells use teleport instead. if you refuse to use both, use teleport and clairvoyance, or heal + ignite.

Here is a recent game which exemplifies a slight modification in which i pushed banshee's veil in front of Randuins omen and spent the game tanking for my team. although i did not get many kills myself, i also did not die much (except for a gank, an accidental tower dive, and an attempted 1v5 haha) The color coded images show me(veloriumcamper is highlighted red and the enemy rammus is also highlighted red. our team's main carry is in orange, twitch, who spent the game running around, ganking where possible and generally killing everything. Our secondary carry is in yellow, miss fortune. she spent the game following me around trying to killsteal me on purpose. no lie. she told me. green, is lux. my laning partner at the begining, i gave her 6 assists early game with effective use of taunt. she was fed early but really didn't make use of it as she did not know how to build well, especially once the laning phase ended. Annie was laning with miss fortune against ryze and brolaf, and had a very hard time during much of the game. she eventually decided to sit at spawn eating a hamburger and texting until major team fights started, which she spent clearing lanes to try to catch up, and running around assisting.

Following this if you're not a scrub ;)