The most 'interesting' investment in Bill Ackman's portfolio that 'no one has really noticed'

Activist investor Bill Ackman, CEO of the $18 billion hedge fund Pershing Square Capital Management, says the most interesting investments in his portfolio right now are the mortgage originators Fannie Mae and Freddie Mac.

The 49-year-old activist investor was on a panel on Wednesday with fellow activist investor Nelson Peltz, a friend of Ackman's and CEO of Trian Fund Management, at the CNBC/Institutional Investor Delivering Alpha Conference at the Pierre Hotel in New York.

"Mad Money" host Jim Cramer asked the pair for their single best idea.

Ackman said his was Fannie and Freddie but "no one has really noticed." By the way, he presented on Fannie and Freddie at the Sohn Conference in 2014. At that time, Ackman said he saw the stock price ranging from $23 to $47 in the future.

According to Ackman, this investment offered "the most upside and probably has the most downside."

Fannie and Freddie needed massive bailouts amid the financial crisis and were taken over by the government. As a shareholder, Ackman thinks the government will eventually move toward a less regulated Fannie and Freddie.

He did acknowledge, though, that there was a "lot of risk" in the trade.

Peltz said he had two new positions that account for one-third of Trian's portfolio, but he was not ready to announce them. He gave some hints, though. One is industrial, and the other hasn't been categorized yet.