I've been waiting for this fight to be made!
If speed is not equalized, then Meliodas simply speed blitzes. Nothing more to say.
Assuming speed will be equal...
Meliodas advantages: superior Regenerationn, harder to kill due to seven hearts, Full counter reflection (but iirc, it only applies to abilities, so physical blows do not get reflected), and Revenge Counter as a clutch. Also, he has thousands of years of combat experience.
Doflamingo loses range advantage from this, as using his DF offensively may hinder him against Meliodas, so he'd have to bind him in order to land a blow with his DF power without full counter kicking his ass.
Doflamingo's Advantages: Precognition, intense stamina and endurance, Environmental manipulation via awakening to affect the battlefield, the ability to manipulate his opponent via parasito. That's about all I see.
At first, I was starting to think that Meliodas would take this with high difficulty, but now i'm not so sure.
@Dark - Meliodas's durability is Likely Island level since Large Island level attacks were capable of harming him, but he could fight against Island level characters quite competitively. Doflamingo would be capable of dealing damage to him, and vice versa.
@Rex - Likely Island level isn't always < Island level. the Likely is placed there due to it being uncertain if the character is that tier, but Doflamingo has many powerscaling feats that would probably put him at Meliodas's level physically.
I am certain that this fight would end up being mostly hand-to-hand due to Doflamingo's threads being useless on their own until he can actually catch Meliodas in his threads.
It would totally end up being an epic fight, but unless Meliodas acts stupid and gets himself in a pickle and struck by God-Thread, I see him winning due to superior Regenerationn and Revenge counter as a clutch.
Meliodas wins High-difficulty