Hello
I've mained Rammus on OCE since s3. Even with the huge jungle changes and removal of spirit of the ancient golem (never forget) he is still a viable choice for tank jungle in the current meta (especially against AD heavy teams).

Pros:
Good against AD champs
Good engage
Massive movement speed buff
Great pushing power at lvl 6

Cons:
Not so good against spellcasters
Slow camp clear (early game)
Long cooldown on movement buff
Not as good at peeling as other tanks

Why no Randuins or Frozen Heart?
Randuins and Frozen heart both give massive armour however they also reduce the attack speed of enemies. So when comboing Q -> E -> W the target (hopefully enemy adc or bruiser) they will do less damage to themselves.

Why Abyssal Sceptre?
Abyssal Sceptre is the best MR item for rammus period. Its aura reduces enemy MR by 20 increasing all magic damage to enemies within range (all taunted enemies). This includes damage done from thornmail and DBC. The extra AP adds damage to tremors as well which is always handy.

What boots to buy really depends on your play style and how your team is doing. Just note that nothing is funner than home guard teleporting then activating Q.

Liandrys torment and Rylais crystal sceptre both proc off DBC which can be handy especially against anyone who tries to kite you.

Both health and armour are necessary for tanking the adcs.

- Base defence runes for early game protection against mobs.
- Magic pen for increased return damage.
- Movement speed for ganks.
- MR for late game to protect against high AP champs.

First level goes to DBC as its the best skill for camp clearing. Second goes to Powerball to move from camp to camp. Third goes to DBC again for extra protection on your ganks. Then start dumping all your SP into your taunt and ult to get those ganks down.

Oppression is the main mastery here as it further reduces damage to you from taunted enemies.

I also like runic affinity to help with clearing the camps early.

In team fights Rammus really has two roles depending on who his team is.

If his team has a Blitzcrank or other tank that has good peeling abilities Rammus should stay with that tank to instantly lock down the enemy adc when they engage because the other tanks abilities are on CD.

However if there is no other tank on his team Rammus has to peel himself. Because all his abilities have short range rammus has to get right into the enemy team to do anything which can be difficult if the enemy team has lots of CC. Apart from getting banshees veil you can use powerball to bait the enemy team into using their CC and then dodging with the boosted movement speed. Then engaging with the Q -> E -> W -> R combo focusing the enemy adc will shut down their adc and do half the health of any other enemies you can stick on.

Smite for jungle.

Flash for chasing down more than running away. (You can use flash while rolling)

Teleport for homeguard massive movement speed anywhere on the map.