Gains in shares of Apple Inc could take the world's largest publicly traded company to a market capitalization of perhaps as high as $1 trillion, some of the country's biggest investors said on Monday.

"It's an attractively priced, double-digit returner," said Steve Einhorn, a top executive at Leon Cooperman's $10 billion hedge fund Omega Advisors Inc.

"We own it, and we like it," he said, speaking at the Reuters Global Investment Outlook Summit in New York. The company could reach the trillion-dollar mark, "eventually," he said.

Apple's current market cap stands at about $670 billion, according to Thomson Reuters data.

The company remains undervalued, said billionaire activist investor Carl Icahn, who as recently as last month again urged the company to buy back more shares using its $133 billion cash pile.

"Apple is one of the best companies I would say of the last few decades," Icahn said on Monday. Because Apple is undervalued and has a pile of money, "it's like a no-brainer to buy your own stock back." With $135 billion in net cash and marketable securities on its balance sheet, Apple can execute on both stock repurchases as well as new product investments and products, Icahn said.

Icahn owned almost 53 million shares of Apple as of the end of the third quarter, according to a regulatory filing last week.

Icahn stopped short of placing a fair value on Apple shares. "That's a question that is extremely hard to answer," he said.

Shares of the tech company have gained about 42 percent this year, helped by strong sales of the company's flagship iPhone.

In June, the company split its stock seven-for-one and in April it raised its share repurchase authorization to $90 billion from the $60 billion announced a year earlier.

Apple's market capitalization could reach as high as $1 trillion next year, said Michael Corcelli, head of the Miami-based Alexander Alternative Capital.

"Absolutely, without a doubt," he said, citing the company's popular tablets and phones going into the holiday shopping period, when retail sales generally spike.

"Right now Apple is the most obvious thing that has to go up," Corcelli added.