Sucker Punch. Even if you're going to think Iron Head is better because Max Steelspike raises your Def while Max Darkness does nothing since its physical, Sucker Punch is very helpful, and considering its a helpful priority, and helps a lot, as Fizz told:
Most coverage attacks are not really concerned about covering weaknesses to begin with. More typically, you simply use whatever allows the Pokemon to hit the largest range of targets (especially if those targets wall your STAB, like Dragapult does in this case). There's often overlap between your defensive weaknesses and gaps in your offensive coverage, though.
Of course, Dragapult isn't common in Ubers, but as ★~ProfDelldell~★ told, checking Calyrex Shadow makes it useful, since Calyrex does outsped it, and revenge kill a lot of things faster than you. I'm assuming Ubers, but if it's OU, then Sucker Punch also beats Dragapult.
Hope this helps!