This is just a quick view on Darius and what I've found is very affective.

I started the build off with a Doran's Blade because it's a good starting item for AD characters. I then buy boots because as you know if you've played him he seem's to be one of the slower melee champs. After boots I buy a Vampiric Scepter so he has better surviveability in lane. After that I head too Berserker's Greaves which really helps him but some damage out there while also help keeping his health up with lifesteal. Then I buy a Sword of the Occult. This really helps with the damage output and is great with his ulty do to the executions it can perform. I then buy a Phantom Dancer Because it helps build stacks on the sword of the occult and makes it so fewer champs can outrun Darius. I then buy a Crit. Cloak for some more damage output. After that i get Warmogs Armour which really helps with his surviveabilty also making people less likely to target him due to the heavily marked health bar. Then I purchase Atma's Impaler. Building Atmogs as Darius is great because it lets him take tower hits and melt some champions really quickly even if they're hugging the towers. Finally I buy The Bloodthirster. This item closes up the build giving him the bonus lifesteal is great for keeping his health up and the extra damage just makes it that much easier to kill buildings and champs.

With this build Darius' ulty does a max damage of a little over 1000 and about 550 just normal. This destroys other champs and is great in team fights especially if you keep hitting killing blows with it.