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.
I know drop shipping can get a bad reputation, but when done correctly it can be an effective way to create a passive income streams. If you can program well, I'm not sure if I'd recommend it over a SAAS based approach. You'll likely be able to make more money with SAAS as you're creating your own products vs. selling an existing one. But the long-term upkeep is generally less for eCommerce vs. SAAS (guessing here, as I've never run a SAAS business).
Margins are a bit lower with DS, so it does take a while to build some organic traffic and marketing buzz. But once that starts to gain some traction, it's not too much work to get a team in place and automate things. And if you're willing to put in the time to make a high-quality site that addresses customer questions and buying hesitations up-front before the sale, it's even easier to automate on the back end for support.
But you do have to be careful about niche selection. Because drop shipping markets are fairly competitive (low barriers to entry), you have to be really selective about what nniches you get into. Specifically, I like to see really confusing niches where I can add lots of informational value, plus lots of accessories I can sell to increase overall margins). For anyone who is interested, I wrote a blog post on the three crucial things I look for when picking a niche for eCommerce and drop shipping:
I got pretty lucky with them. One one I distinctly remember I bought for less than half of what the people losing it (short sale) still owed. They were all in pretty good condition, I just replaced the carpet, painted, made minor repairs, and replaced appliances.
the main risk here is inflation risk. if inflation in the country takes off your 5% yielding (likely 30% leveraged) bond fund is going to start to see its real return get eaten away quickly.
the alternatives are equities though. and right now the stock market is pretty scary to me.