Google has knocked Apple off the top perch after only six years. it has been approaching for some time with clear profitability and Apple's reliance on hardware - specifically the iPhone a risk to long term sustainability.
What company is next in line to challenge Google if Apple cannot?
Some form of monopoly in a profitable global macro market plus the ability to innovate every day life are a must.
Apple became the United States' most valuable company in 2010, when it dislodged Microsoft. Microsoft in turn overtook IBM two decades earlier.