Google says its updated search method can now understand superlatives such as tallest and largest, for example. And it can provide specific information for a point in time: "What was the world's population in 2010?"
Lastly, Google is putting the meaning of multiple items together through searches with more complex combinations. You can put various requests together and Google will understand the intent: "Who was the U.S President when the Angels won the World Series?"
In a case such as that, Google is actually searching for two completely different data points that are tied together though some relationship such as time.
In terms of advancements, I'd rank Google Search and Cortana as leading the pack. It also helps that they're both available for multiple platforms: Chrome, Windows, Mac, iOS, and Android, for example.
Siri, on the other hand, isn't just limited to Apple devices but it's also more limited in terms of functionality. After debuting in 2011, I thought Siri would be a stronger contender in this area by now, but it seems to have only improved incrementally compared to the others.