Finishing Item sets, Darius's arsenal

Hey summoners :) just a quick question for y'all.

I main for almost all my top lane problems and i usually do really well, but i only have one issue with him.... What to finish his item set with????

My "go-to" build for him is this (in exact order): then either, or .... but there is always that last slot i don't know what to fill with :\ .

This last slot i usually save for my item that will attempt to counter what ever the main damage output is, but i can never really settle on anything that will finish out the build if i snowball really hard and just have all the extra gold XD. So if you can please fellow summoners, assist me in finding my final "go-to" item so i can wreck my opponent on a regular basis :)

Hopeful champ contender,

Buddhabob9