PROS:

Great CC and teamfight potential
Can surprise many top laners with his damage
People forget to take MR into him
A tank for your team
Good waveclear with his W and E
His Q hitbox is so ****ing broken

CONS:

Weak before level 3
If you die early the lane is often over
Laning often relies on baiting out his enemies
Mercury Tread's hard counter him
Quite a low mana pool

Conqueror:
Conqueror is great for Amumu top, as it stacks so easily with his W, E and Bami's. All you need is the Q > E > W plus a few autos combo and it will be almost fully stacked. This allows Amumu to take extended trades and punish enemies if they stay close to him for too long.

The rest of the Primary Runes:
Triumph and Tenacity are standard for top lane and a no brainer. You can take Coup de Grace or Last Stand (I always take Coup de Grace), to increase your final bit of burst damage to the enemy.

Domination Secondary Runes:
We take Domination Secondary, because Resolve Secondary is for people who are bad lol. Also Cheap Shot, Taste of Blood and Sudden Impact are all really good on Amumu. I usually take Taste of Blood, but Cheap Shot can be better if you are looking to engage with your Q > E > W then back off. ToB is better with the E poke playstyle I like to play. Ultimate Hunter is a no-brainer secondary as - although it can be hard to stack in top lane - it is Amumu's ultimate that really makes him a viable champion.

Use your E to farm the first 3 melee minions and try to hit the enemy top laner as they go for cs.

Save your second Q and ult for the last bit of burst.

Always combo ult > Q in lane, as it guarantees your Q landing.

You can Q > E > W, and then run away to get free damage and bait the enemies abilities.

If your low HP, try to bait enemies under your turret then use W > E > R > Q to clear the wave and CC lock them.

Tell you bot lane to get a deep ward so you can TP, and try to hold your TP for teamfights.

Thanks for reading my guide on Amumu top! I hope you have fun trying out this playstyle and it enables you to climb the LOL ladder (although it may not). If you have any questions or things you think I should add, let me know in the comments and I will check it out.