Alphabet briefly became the world's most valuable company after its February 2016 earnings report.
Alphabet was created in 2015 as the holding company for Google and several other businesses. Its first earnings report as Alphabet, released in February 2016, showed strong advertising performance and helped push its share price sharply higher. For a short period, Alphabet's market capitalization exceeded Apple's.
The ranking was not permanent. Market capitalization changes with share prices, and Apple soon regained the lead. Alphabet's result nevertheless demonstrated how quickly the largest-company ranking can change after a major earnings announcement.
A common mix-up is calling the company Google in this question. Google remained Alphabet's principal operating business, but Alphabet was the publicly traded parent company used for the market-cap comparison. Alphabet's largest revenue source has historically been online advertising, particularly through Google Search and YouTube.