My "best" passive income source would be subscription revenue from SaaS businesses. I run several, with no employees, with growth from existing marketing and word-of-mouth outpacing churn, and no active management needed other than answering customer e-mails. Everything that can be automated is automated, from backups, to monitoring, to lifecycle and dunning e-mails. Improvely (https://www.improvely.com) is the newest, just over a year old now, and passed $10k/mo RR not too long ago.
If that's still too much to qualify as passive, the stock market contributes the next largest chunk of revenue. Of the money I put in equities, 2/3rds are in index funds (primarily total market stock and bond indexes), and 1/3rd in hand-picked tech companies. The market's been good since the recession crash; better than a 25% annual return.
My most passive income is from revenue sharing affiliate/referral agreements. I work with a lot of small businesses with websites, and refer them to vendors for merchant accounts and advertising services as both publishers and advertisers. For all three, I found good companies to work with that pay a percentage of referrals' spending for the life of their accounts. There are businesses I referred for credit card processing almost 8 years ago I'm still getting a monthly commission check of several hundred dollars a month for, each.