Billionaire investors reveal their best idea

Housing still a good investment: Blackstone CEO
Housing still a good investment: Blackstone CEO   

Major private equity fund managers, including several billionaires, gave their best investment idea at the Milken Institute Global Conference.

Speaking in Los Angeles on Monday, each revealed what they thought was the surest way to make money by investing in private assets.

Read MoreTrust robots with money, not 'emotional' humans

Steve Schwarzman, chairman, CEO and co-founder, Blackstone Group: European real estate. "[It's] still is a very interesting idea."

Leon Black, chairman and CEO, Apollo Global Management (he gave two): Energy-related investments and European financial institutions.

Jim Coulter, CEO and founding partner, TPG: Health care. "There's a huge disruption going on and it's not attached to the rest of the economy."

Robert Smith, founder, chairman and CEO, Vista Equity Partners: 3-D Printing. "That whole technology, that sphere of influence, will actually change a number of industries ... and will be a massive disruption."

Jonathan Nelson, founder and CEO, Providence Equity Partners: Premium content and digital distribution.

Opening up PE: Schwarzman
Opening up PE: Schwarzman   

The investors otherwise urged caution in investing given relatively high asset prices.

"We are living in a high-priced environment and one has to be highly selective in terms of what one buys," Black said.

Read MoreBig money: 'Awfully dangerous' to take risk now

He noted opportunities in lending to companies in need—"credit" investing—where private equity firms like Apollo could fill the void of reduced bank loans.

Schwarzman added that private equity funds such as Blackstone are at least set up to wait for the right opportunity over several years.

"We're like basketball teams without the 24 second clock. We don't have to shoot," Schwarzman joked. "I like easy shots. ... You can do that if you hold the ball for four minutes and don't turn it over."

Read MoreBlackstone CEO: Housing makes a 'good investment'