With three major smartphone AIs giving out answers, it's hard to say which one is the best. In a study that asked all three major AIs over 3000 different queries, Google Now came out on top.
This study, conducted by Stone Temple Consulting, is based purely on knowledge queries, not a comparison of personal assistant capabilities. That being said, all queries were conducted using voice commands so that there could be a direct comparison of not only how well information was retrieved, but also how well the AIs could understand your queries.
When it came down to which AI was able to give the most "enhanced results" -- answers that give more information and knowledge snippets -- Google Now was able to do so 58% of the time. Siri gave enhanced results 29% of the time, while Cortana gave enhanced results 20% of the time. For the most important aspect -- the results completely and directly answering the question given -- Google Now answered completely 88% of the time, with Siri at 53%, and Cortana at 40%.
This means, that in general, Google Now was able to directly answer the question with the information it provided immediately, with no additional digging required. Siri and Cortana often produced helpful results, but some additional digging was required to find the correct and complete answer. This doesn't necessarily mean that Google Now is the best AI overall -- and all three will be changing considerably over time -- but for the time being, Google Now could be considered the most knowledgeable. The entire study is worth a read, and you can learn more at the link below.
The Great Knowledge Box Showdown: Google Now vs. Siri vs. Cortana [Stone Temple Consulting]