Warren Buffett claims that the best business that he was ever in was installing pinball machines in barber shops, then splitting the revenue with the barbers. See https://www.cnbc.com/2018/06/19/warren-buffett-bought-a-25-p... for verification.
Expert networks are an example of a side-gig which would compete with traditional "passive income" stuff for talanted tech folks.
He's had far more income from other investments. But he turned $25 into $2000 in 3 years. That's around 331% profit per year, compounding annually. Given that he likes to invest in relatively mature companies, I can easily believe that no other investment ever showed a greater annualized return.
(Of course this calculation discounts his personal investment of time to run the business.)
You run out of barbers rather quickly.
Away from this passive income has a terrible tax profile--unless you employ leverage (see above).
> As Buffett told Gates in the Omaha candy shop: “It was the best business I was ever in. I peaked very early in my business career.”