Doflamingo mid-high diff. Sanji's fire couldn't burn Doflamingo's strings away, I doubt Ace can either. There's still a possibility that Ace can turn into fire, deform, and escape whatever method the strings bind with, but we don't know if it can definitely happen since we don't quite know the limits of Doflamingo's control.
Seeing that Doflamingo has greater speed feats than Ace too, I just don't see Ace winning. Teach mentioned that it was a while since Ace was hit physically, indicating he relies heavily on his Fruit's intangibility, so Doflamingo's Haki-laced blows can knock Ace off his feet and cause him to stagger, which could make all the difference. Seeing as Doflamingo could take Red Hawk, a fiery attack with Haki laced into it with no significant injuries, then I just don't see Ace harming Doflamingo that much. Plus, Doflamingo can make it two vs one with Black Knight, perhaps even multiple clones. Bird Cage can just be bad news for Ace.