Leiter's been running a poll, and Brian Weatherson has some commentary. He voted for Lewis, and I voted for Carnap, but his post is about why Russell has been doing so well. I'm actually a little surprised that Lewis is doing so well, though I think he's a perfectly defensible choice.
It seems that part of the reason Weatherson is surprised at Russell's showing is that the project of Principia Mathematica ended in failure. I find that very difficult to evaluate. It made contributions to the state of modern logic, and I guess I tend to think that modern logic is a truly enormous philosophical achievement. However, it's difficult to know how to assign credit for it. It's amazing how much was already set out by Frege; most of what comes afterward could be seen as just clarifying and patching a few mistakes. But clarifying and patching mistakes is perhaps not trivial in this area. How much did Principia move beyond Frege? It did, I suppose, have some mistakes of its own. I'm not sure exactly how to evaluate it, but I am skeptical that the failure of its official stated project is a particularly important criterion.
I suppose there are similar issues with Carnap. I rank him highly both because of his contributions to the unfolding story of modern logic, and because I think his philosophical attitude toward logic was entirely correct. But how much did he really add, as opposed to clarifying? And how much did he clarify, if, as I tend to think, so many of his contemporaries and near successors misunderstood him?
Maybe that's an argument for Weatherson's choice. There is no question that Lewis clarified many things. But I guess I still think the same is true of Carnap, and of Russell, and that perhaps it is only less obvious for them because what they worked on was so unclear before they got to it that even quite substantial progress still left plenty of murk.