Darius is kind of op champion with true damage refleshing ult. This is why he doesn't need full damage build. So i made one offtanky build on Darius. I hope u will enjoy.

At start u should take boots/4pots for more sustain and mobility.
Buy any wards in during game.
Then 1/2 doran(s) for stronger early game.
Then buy ninja tobi or mercury's treads -> depends on champ you are fighting on top.
Next item frozen malet for perma slow and HP's for atma's impaler.
Then atma's impaler for some ad, crits.and some armor.
Last whisper becose i am pretty sure they will buy some armor and it gives some ad.
Frozen Heart for better sustain in teamfights and mana becose ult cost 100 mana and u can use this ult 5 times in a row.
Last item Bloodthirster gives lot of AD,LifeSteal.

If you will use ult make sure, that you will kill him -> I am sure u don't want to wait atnother 100 seconds for CD.
If you wanna deal most damage with your ult stack your passive on your enemy target (maximum 5).
Don't set your E skill on ''smart cast'' becose it's important to know your range on skill.
Spam your Q skill in early level's on enemy and it gives you advantage on lane.
And last advice is ''DON'T BE GREEDY BRO''(even when u got lot of kills stay with team).

I set masteries on 21/9/0 becose u will be agressive on lane and it gives you more AD than basic(it's what you need). In defense some armor/magicr. and health regen for a bit better sustain.

Classic runes for solo tops.(Some AD, Armor pen., armor and magicr.).
And same with spells also calassic (flash for escape, ignite for fighting).