I have never successfully found a Morkekaiser build that I enjoy whole-heartedly.
There was always too much Ability Power and not enough survivability.
That Is why I want to post this build in the hopes that you will enjoy not only owning opponents, but pwning opponents.

For runes I always go for Flat Health quints simply because it fits on any champion and it gives Mordekaiser extra durability that he absolutely thrives off of.

Because Mordekaiser has no mana pool, a large number of runes are cut out. I tend to go about my runes with 3 goals-

1. Cooldowns for his ultimate
2. Magic Penetration for Mace of Spades and Siphon of Destruction
3. I use Crit chance to further improve his melee attack

As far as masteries go, I follow a 9-20-1 build. I pick up cooldown reduction in the offensive tree as well as extra AP and crit chance. I think that this gives Mordekaiser a well-rounded offense that grants your foes a challenge.

In the defensive tree, I tend to lean towards as much durability as possible picking up extra armor,dodge, and magic resistance. This assists in making Mordekaiser more durable early game.

I don't like a 9-0-21 mastery on Morekaiser. I don't treat "Mord" as a caster, and I never will.
That is why I only pick up the Ghost talent improving his chasing and escapades.

I have heard of people leveling Creeping Death first. That doesn't work on this build. I chose to level Mace of Spades first and switch between it and Siphon of Destruction.

Reason: I do this because it helps extremely well with your shield bonus and charges it up well making you more durable than if you were leveling Creeping Death.

Of course you grab your ultimate whenever it is available at level 6,11,and 13

Level Creeping Death last. This will give you support and an AOE Defensive Mechanism that can be used on you or on your teammates. I like this skill because it makes enemies run when you're only a little more threatening than before. I get a chuckle or two for this reason.

Wooooooo hoooooo. Mordekaiser is not one of the most item reliant champions. I think that you will find my build in no way to be short on ability power.

Because Mordekaiser is fairly "squishy" per say early game, I pick up health regen and armor as well as Magic Pen with the Sorcerer's Boots, Sunfire Cape, and Regrowth Pendant. I then like to put on a little bit of health regen and more resistance with the Force of Nature. I cannot stress the importance of this item. Mordekaiser is constantly using his spells especially because of cooldoen reduction. This drains his Health moderately fast; so getting extra resource regen is a :)

Whenever going for any defensive items with the Giants belt, I find that it is best to buy the belt first. I would do this on both the Sunfire Cape and the next item, the infamous Warmog Armor. This item is insanity at its finest on Mord. It gives him a ton of health and makes him unkillable. This makes him an efficient tank and an all-around good support.

If you have time I stack two more health items, I stack the Spirit Visage and Rylai's Crystal Scepter for Ap and Defense.

These items satisfy me and I hope that they will satisfy you because you'll be pwning it up with the Master of Metal.

In summoner spells I grab Ghost and Ignite.

Ignite: I use Ignite because it works so well with Mord's ult. It gives you a much stronger DOT than he would normally have.

Ghost: I grab Ghost because this improves Mord's chasing ability and his escape ability. Mordekaiser has no stun so this is very important.

Spells to NOT Use

Promote,Clarity(duh),Fortify: I find this to be useless on Mord

Spells that may be used to your advantage:

Cleanse is always good because Mordekaiser is always targeted first. This can help him escaped from stuns, fears, and prisons.
Exhaust would also be beneficial because of Mord's lack of a stun

I like this build. This is also my first build. I would like to thank anybody who reads this because I love the game League of Legends. It is truly my favorite MOBA by far, and I hope that this build could perhaps enhance your gameplay.