Not Updated For Current Season
This is a DPS Mordekaiser.
First of all, I start with a Doran's shield. Good hp start off and hp regain and armor for survivability. Ninja tabi, for dodge since my masteries are defense. I get this also, because I'll be getting a Phantom Dancer. This'll give me maximum dodge.
Phage - As you know, Mordekaiser doesn't have any slowing abilities. This will help non-squish him and give him some dmg.
Lifesteal - You all may wonder why I get vampiric scepter. As you know, Mordekaiser is a good farmer. If he builds this into a bloodthirster, he'll have a lot of damage. Lifesteal can also help replace his hp regain.
Phantom Dancer - Great atk speed, DODGE, and crit chance as well as movement speed.
Your last item could be one of these ; Atma's Impaler (Armor + Crit chance), Sunfire Cape, or Guardian Angel if you have a hard time surviving.
You may wonder, why I don't have ghost. Once again, Mordekaiser does not have a slow. His masteries can make his exhaust reduce armor and magic resist. Which is good for 1 vs 1ing.
I often like to hide in bush when getting chased at low hp and if the other enemy is low in health, I'll exhaust ignite and boom! They die.
I get my E maxed out first for maximum harass, armor, and damage.
The reason I don't get Q is that it doesn't do much at low levels.
Your W skill can replace a sunfire cape, because of the aura dmg and the armor and magic resist.