Not gonna lie, Darius is the champion that made me quit top laning after I came back to the league. Of course, I was pretty ****ty and never took the time to realize how easy it actually is to beat the dunkman. The key is to treat the matchup as if it were a boxing match; you gotta stick and move, Darius counters those with no disengage and no sustain. A dunkman without his pull is worthless. Some of my favorite champions for the matchup are as follows:
Riven: Start cloth+5, work into an early vamp scepter. Bait out the pull, dash in, stun, use up your passive stacks, see if you can get a few q>aa's off and use whatever q's you have to disengage. Rinse and repeat until level 6 and you can harass him down till your ultimate does enough to execute him.
Rumble: Start cloth+5 and work into a revolver. Play super passive till you have 3 ranks in your flame spitter, then you can bait the pull and jump onto Dunkman's face with flamespitter and disengage with the unbearable slow from your harpoons and shield a good portion of the bleed from affecting you. At level 6, if he's below half health, bait the pull and drop your ult in a way that Dunkman has to either fight you in your ult or run away through your ult. If he fights you in your ult, he's a dead dunkman. If he runs through your ult, tag him with a harpoon and roast his beef with your flamespitter as he runs away at less than half of normal speed.
Trundle: Start cloth+5 into early vamp scepter. You have innate sustain from your passive and as most Dunkman's build full tanky with maybe a frozen mallet for damage, relying on their ult for all of their damage, you're going to have a field day stealing all of Dunkman's AD so he can't do **** to you. Really is an unfair lane for Dunkman.