After never being a dominant force in personal computers, Apple surged to the top of the S.& P. 500 by transforming the cellphone into a multitasking smartphone, arguably the single most important technological advance so far in the 21st century. It rolled over vaunted rivals like Nokia, Motorola and Research in Motion with a combination of brilliant technology, dazzling design and shrewd marketing backed by the singular vision of its late founder, Steve Jobs. “Everyone truly needs it,” Mr. Cihra said of the smartphone. “It’s the most transformative piece of technology in our lifetimes.”