Hi, I'm a master rank player in TW server,summoner name Rocket_Raccoon,
(due to work shifts cannot constantly play league, always gets kicked out of master by system :P )
always seeking non-mainstream, fun, also viable builds to play league.

top is a viable build with ultimate fun,
he is the first champion that got me into the challenger rank.(before master patch)

Some of my friends already got great success with him,
and I believe many of you can get even higher elo than me with him,
for I'm actually really bad with micros and mechanics.

Pros

Tanky

Dominates mid and mid-late game

Massive AOE damage

Easy killing carries and assassins

Great mobility

Map control

Keep feeding resources to teammate

Ultra fast wave clear as game goes on

Scale with items well

Cons

Lose opponent in CS most of the time

Might behind in levels if make wrong decisions (ex: early dragon)

Decision making are important and can be difficult (ex: recall timing after battle)

Teammate might flame you because you didn't take teleport or you are behind in early CS

Not as tanky as defensive Rammus when everyone finished full build

99.999% of the times I go for Flash Ignite.

Exhaust and Teleport works fine I guess,
but I think Teleport isn't worth it with this play style.

Rammus top, for me, he's already got enough MASSIVE defensive stats from his Defensive Ball Curl and runes,
and plus,
this is a very offensive and aggressive play style,
so in my opinion he best fits 21:9:0 deep into offensive tree.

<note>
Yeah I know other rammus top players go deep in defensive,
just think not worth it if you can manage to instant kill carries or shred over half of the health of all enemy team with ease and still SURVIVE.

Runes provides early game survivability.
(besides the Greater Mark of Hybrid Penetration)
and the main goal for rammus's runes is to get the most effective health out of it.

Laning V.S AD based Champions

Greater Mark of Hybrid Penetration
Greater Seal of Armor
Greater Glyph of Scaling Magic Resist
Greater Quintessence of Armor

You can also take Greater Seal of Health,
still prefer Greater Seal of Armor most of the times thou.

Laning V.S AP based Champions

Greater Mark of Hybrid Penetration
Greater Seal of Armor
Greater Glyph of Magic Resist
Greater Quintessence of Health Regeneration

I take Greater Seal of Armor because most of them still auto attack.

You can also take Greater Seal of Health and Greater Quintessence of Magic Resist against opponents that does not depend on auto attack, but it's rarely needed.

Get Puncturing Taunt at level 3 if needed.
For Examples:
Helping your jungler ganks, having kill potentials.

Start up items and boots depends on your opponent,
making right decision on build path definitely helps.

For Examples:

Shyvana seems like an AD based champion,
but she's actually an AP based champion WITHOUT CC in the laning phase,

Darius seems also like an AD based champion,
but his passive Hemorrhage is AP damage,
and his Noxian Guillotine is true damage.

After buying boots and rushing Enchantment: Homeguard
Laning would be meaningless for rammus.

You can control early dragons, gank other lane, even teamfight with this,
and recall rush back to top in 25 second.
(recall included)
What about the lane?
you can always use your ULT or simply a WQ combo whenever you like
to push a whole wave and recall.

To be more aggressively,
you can kept forcing the endless trade with your opponent with all your skills and recall with full health, all depends on you.
(clear waves at the same time)

And after Sunfire Aegis,
you can out trade almost every opponents,
winning fights or even get kills would be at ease.

At this point,
you shall be feared.

You are a
TANKY INITIATOR BRINGS MASSIVE AOE DAMAGE THAT CAN INSTANT KILL CARRIES AND ASSASSINS.

or you can just sit back and protect your carries :P

Combine builds with Situationals,

You are ready to deal with everything.

As Beginner

Before hitting 6,
try to focus yourself taking cs and not get harassed to much,
your main goal is to push the lane or pulling it into your turret,
Spiked Shell and Defensive Ball Curl would allow you secure all minions under the turret,
and recall whenever you like if the lane is pushed.

As Veteran

you can start to try harass whenever your Defensive Ball Curl is up,
Defensive Ball Curl and auto attack combine with Powerball would surely wins the trade,
(After rammus buff last patch this just became even more easy)
Puncturing Taunt is used when the enemy tries to escape,
if the enemy is incautious,
you will always have an early kill before hitting 6 or at 6.

This is the playstyle that it doesn't matter if you can win your lane or not,
(rammus laning phase is actually surprisely strong)
You may force the enemy fight every fights they don't want to.
You may abuse your early homeguard and stomp other lane from any direction of the map,
and get TURRETS!DRAGONS!and overly fed TEAMMATES!
You may 1 vs 1 any fools who dare to fight you after getting your Sunfire Cape,
and ALWAYS deal massive damage in team fights.
YOU ARE AN ASSASSIN DISGUISED AS A TANK!

In some games you may ask your mid to help you steal the enemy's upper jungle buff,
your mid would loss 3 melee minion's experience,
but the enemy's jungler would be in a bad position and forced to go top and get delayed.
after clearing the buff hitting level 2 with your W,
recall and Q back to lane,
you will be POWER OVERWHELMING.
(high risk high reward)

After level two of Defensive Ball Curl you can always find chances ran to dragon and finish it with your jungler.

Sorry I haven't setup stream before if you are asking,
but here is one from enemy team side of view.
Play Video ***the video is not mine***

I'm Rammus in the other side,
this shows no matter how much behind your early game are,
you can still be the CARRY of the game if you make right decisions.

This guide works from bronze to challenger,
taught many of my friends and all succeed.

~MAY YOU FIND HAPPINESS IN THE BLESS OF THE HOMEGUARD~

edit:
please leave comments or upvote if you like the guide,
so that I know this is worth your time.

1/13 just updated the masteries, didn't take the flat AP just for some other points,
but it still works fine.