Bill Ackman reiterated his support for nationalized mortgage lenders Fannie Mae and Freddie Mac, during an appearance Wednesday at the Delivering Alpha conference.
Ackman has talked up the two lenders before, but said "no one's really noticed." Fannie is up about 11 percent in 2015 and Freddie is up nearly 4 percent.
"It offers the most upside, probably the most downside of anything we own," the head of Pershing Square said. He believes the downside scenario is unlikely.
The federal government bailed out the two government-sponsored enterprises during the financial crisis.
"This cannot become a precedent where the U.S. government can come in and unilaterally take 100 percent of the profits of a private corporation forever," he said.
Fellow activist investor and head of Trian Fund Management Nelson Peltz took the stage with Ackman but was more circumspect about his best investing idea.