Which product is most important to Google? What can they least do without?
They rocketed to Number 1 in search by using a smarter, revolutionary algorithm. Back in the day, they sent users to those sites with the most inbound links, making the (correct) assumption that the sites with the most inbound links were the most relevant or authoritative.
Today's "digital natives" (that was typed with an eyeroll, ok?) would laugh at the way Yahoo used to manually categorise web sites into lists of groups of groups of lists. It was more like searching the Yellow Pages - another institution many digital natives might need explained.
As soon as Google delivered a product that did the searching for us by using keywords and relevance scores every other method looked antediluvian.
Well, the Big G's position is equally as precarious today as Altavista and Ask Jeeves were a decade ago. Google's search results are driven by keyword hunting but when we search we seek information and knowledge, not keywords.
The first bedroom-dwelling university dropout to come up with a way to understand what we're asking, and to understand what web pages are saying, will either sell his secret sauce to Microsoft or Google for $Billions less than it's worth, or he'll go on to become rich enough to buy Mars.
Google's greatest strength is search but it can be taken away from them overnight by a 16 year old accidental genius.