A prominent economic thinker who has a particular knack for communicating with the uninitiated, who speaks truth to power, has an excellent grasp of economic logic and principles, and aggressively stands up for a liberal society?
I have to agree with Karl Smith - this is a no-brainer.
Henderson will crown who Henderson will crown, and it won't be Paul Krugman. But then, to me, it won't really be a "modern Bastiat". It will be a "best communicator of libertarianism" prize, despite the objection that that's not what's going on.
OK - let the weeping and gnashing of teeth begin. People are already aghast at what seems to me to be a fairly obvious suggestion from Karl Smith.
I probably should add that if one were to ask this question in the 1920s and 1930s the obvious answer would be Keynes.
UPDATE: Here's an example of one objection from Don Boudreaux, who is one of the last people you should ever read if you want to understand anything that Krugman says, or anything about Keynesianism in general: "Yes, like Bastiat, Krugman is passionate in pressing his position. But Bastiat was above all a (classical) liberal - and one ever-attuned to the unseen. Krugman fails magnificently on both counts. Moreover, unlike Bastiat, Krugman spends far too much effort fronting for a particular political party." I wouldn't be so blunt about Don if he wasn't so nasty about Krugman here.
I've found that most fans of Bastiat's often misunderstand and regularly misapply the broken window fallacy - I've gone over that point a lot here, and I don't think my argument on that should be news to anyone. I also think it's obvious that "modern liberals" are classical liberals and that while libertarianism falls under "classical liberalism" too, that is not the definition of the term as Don would have you think. Modern libertarians would not have done well with classical liberals like Jefferson, Paine, and Smith.