Offensive Dusknoir
Dusknoir @ Leftovers
Adamant: 252 HP / 252 Att. / 4 Def.
-Earthquake
-Ice Punch
-ThunderPunch
-Sucker Punch
I've been using this Dusknoir on the ladder for quite some time and it has performed very well. No one really expects Noir to hit this hard, and it nets quite a few surprise KO's.
Earthquake coming off of 328 attack hurts, and hits most things for good damage. It's real purpose, however, is Heatran. When Noir finds himself up against Scizor, you can bank on a switch or U-Turn (for CB Scizor) to Heatran to absorb a Will-o-Wisp. EQ on the switch OHKO's Heatran easily. EQ is your best bet against Scizor regardless, so unless you predict a switch to something to resist or avoid EQ, it's the right choice.
Ice Punch and ThunderPunch provide you with great coverage and hit some other important threats. Ice Punch hits dragons and other fliers like Gliscor, Zapdos, and Shaymin-S hard, while ThunderPunch takes care of Gyarados. Note that Zapdos and Gliscor may try to stall you out with Roost, but they will have to Roost over and over again, and risk a critical hit or freeze, so it's not futile to stay in and keep plugging away.
Sucker Punch lets you make up for Noir's terrible speed and finish off weakened opponents. Shadow Sneak could be used here as well, but Sucker Punch has proven to be more useful as it hits harder (60 BP vs. 80 BP) and has the same coverage.
Fire Punch wasn't mentioned on the set as the listed moves provide better coverage. It does have a few niche uses though. It can be used to hit Heracross harder than any other move on the set, and it can be used on Scizor if you know that a switch to Heatran is unlikely or if Heatran is absent from your opponent's team. Switching in on a Scizor and Fire Punching is a bad idea, as I mentioned previously, because of the ever so obvious but always done anyway switch to Heatran. Fire Punch is really only needed to beat Swords Dance + Roost Scizor, as it can set up on you and Roost of the damage from Earthquake.
On a final note for this set, Dusknoir has often found himself going blow for blow with other tanks like Swampert, and often, when both pokes are in the red, they will switch out to something to avoid the last EQ and threaten a KO. This may be common sense, but it just seems to happen a lot with this particular Noir. Don't be afraid to make the prediction and Ice Punch those incoming Salamences and Shaymin-S's!
Edit: On the last paragraph, I'd just like to clarify that the reason this happens to Noir all the time is that few expect him to be carrying three or four attacks, as he usually has stuff like Pain Split, W-o-W, Trick Room, &ca. Take advantage of that fact and get the surprise KO's that make this set work.