The iPhone is the most important source of revenue for Apple, and this year it's more significant than ever. The smartphone's net sales crossed the $100 billion mark in 2014. That's more than the gross domestic product of North Dakota and Rhode Island combined and more than some countries, including Ecuador.

Net sales for the iPhone rose to $102 billion in 2014, an increase of 12 percent from $91.3 billion last year, according to Apple's annual report, released on Tuesday. As a result, the smartphone now accounts for over 55 percent of Apple’s sales. The company beat expectations in its fiscal fourth quarter thanks to robust demand for its larger iPhones: the iPhone 6 and iPhone 6 Plus.

Overall, Apple attributed iPhone sales growth to a combination of sales of the iPhone 5s and 5c, in addition to initial 6 and 6 Plus sales.

Apple’s success with the iPhone has driven sales to 169.2 million units, but there has been some concern for the company’s iPad line, which saw a 5 percent drop in revenue this year to $30.28 billion, down from $31.98 billion in 2013. Despite the doubts surrounding iPad and the global tablet market, Apple CEO Tim Cook remains hopeful about the device line. “Over the long arc of time, my own judgment is that iPad has a great future,” Cook said. “I’m very bullish at where I can take iPad over time.”

Apple's price-to-earnings ratio remains at 16.60, 10 percent below the 18.43 price-to-earnings ratio of the S&P 500. In comparison Google maintains a P/E ratio of 28.33, while Facebook dwarfs both with a P/E of 87.74 -- a sign investors expect the company to grow faster than its rivals.

Credit Suisse reiterated its position on Apple with a neutral rating, citing the company management’s “exuding confidence” over the trajectory of its business, following the release of Apple’s annual report. This echoed a “hold” reiteration from S&P Capital IQ last week, following the release of Apple’s fourth-quarter earnings.

Most analysts have a "buy" on Apple, according to data from Zacks Investment Research. Analysts’ consensus also leaned towards "buy" positions for Google and Facebook. In Amazon's case, analyst recommendation has been split between holding the stock and buying.

Despite positive outlook on Apple’s future growth, some still believe it is undervalued, including activist investor Carl Icahn, who owns approximately 53 million shares.

Analysts expect the iPhone to see continued growth into 2015, but industry watchers are also looking to Cupertino's upcoming wearable, the Apple Watch, to drive further revenue gains for the company in the nascent wearable technology industry.