I have two candidates.
I think Google and the Internet means that all information is at the fingertips of everyone and therefore allowing the great and good to go off and do their own thing in a well meaning way is increasingly not an option. Makes the politics of such treaties harder and harder to sustain against insurgent populist campaigns. Voters are less and less willing to trust their elected leaders on such matters and less tolerant of treaties they can't read.
But surely the single person most clearly blameworthy person has to be France's very own Valery Giscard D'Estaing.
After British politicians warned against calling it a Constitution - he went ahead and called it a Constitutional Treaty..... thereby guaranteeing the opposition of the die hard anti-Europeans, and incidentally pushing the French and Dutch to hold their own referendums, killing the first version of the Treaty stone dead and carrying the Reform Treaty in its wake.
Finally he gave this quote about the Lisbon Treaty, which wasn't the greatest help in the ratification campaign:
When the Lisbon Treaty has friends like these, I don't think there's really much of a role for eurosceptic No campaigns.
public opinion will be led to adopt, without knowing it, the proposals we dare not present to them directly