Stone Temple presented each service with 3000 voice queries, and tabulated the results. Not surprisingly, given Google's specialty in search, Google Now was the clear leader. It returned the most enhanced results and complete answer by a wide margin. The study cross-checked the queries with web searches on Bing and Google. Interestingly, Bing returned more complete results than Cortana, indicating that Microsoft could improve its mobile assistant by more fully integrating it with its search engine.
Based on my personal experience with Siri, I wouldn't be surprised to hear that both Google Now and Cortana are better than Siri, but a test of how well one of them searches Google (or Bing) doesn't tell me anything about how Siri, Google Now, or Cortana help me in my day to day life as a “mobile assistant”.