Doflamingo, followed closely by Law.
I don't like Ace but he's overall pretty well-written and the conclusion to his story was good so I'd say he's #3.
WB didn't get a ton of focus but he's a good char w/ a solid story. #4 for me.
Sanji's #5. His writing's often considered good and I'm not sure why. It's true Oda focused on him a lot, extended his backstory and all, but the content's more important than the volume and I just think Oda's been doin a poor job w/ him since the timeskip. He became a caricature of himself and I didn't enjoy his backstory, which could have been better handled imo. Wasted potential.
Robin would be higher on the list if it wasn't for her post-TS representation. Oda completely neglected her after EL and she's barely relevant now.