Bikers has been owning GTA press ever since it launched a littler over a week ago, and with reason. The massive update added a host of new content to Online, including new vehicles, missions and a new business-owning mechanic opening up new opportunities to make cash. While even upon launch the specifics of the businesses were still shrouded in mystery, by now enough time has passed for players to learn the ropes.
Businesses offer long-term passive income, which is why their efficiency hadn't been accurately gauged earlier. Now that players have had a week to track their profits and determine which of the new GTA Online businesses are the best investment, guides have been popping up all around regarding maximum efficiency.
In total, there are five types of business that your character can own, and you have the chance to own one of each type simultaneously. That said, players have already reported management turning into a hassle at just four, so while you can own all five, it doesn't mean that's the best approach (unless you take a page from the book of real world companies, and hire people to play for you while you're doing something else).
The businesses include document forgery, money laundering, cocaine labs, meth labs and marijuana farms. Each of them have a different rate of production and a different profit margin, and the cost of supplies are different as well. That said, we suggested in our Money Making Guide to steal supplies instead of buying them, however this is only viable with 1-2, maybe 3 businesses max.
The Meth and Cocaine labs take the longest time to produce goods, however they have pretty high payouts, coming in at around $GTA 420-430k every 5 hours. The document forgery and weed farm businesses bring up the rear with a ~130k payout. It's smaller than the hard drug labs, but it's ready sooner. In the mid range we have money laundering, giving players a decent balance of speed and profit.

As it stands, coupled with the hybrid crate method, the high-end businesses are the best in the long term, however they require a large investment, as upgrades are an absolute necessity unless you plan on bleeding cash through your little enterprise. The Meth and Cocaine labs take a good long while to make a return on investment, however once they do, the profits will be glorious, and you can go on playing other profitable missions in GTA Online in the meantime.
The best thing about businesses is that they keep ticking down on the timer in the background even when you're not in an MC or busy doing other activities like Heists or running crates as a CEO. This way the high-end businesses offer plenty of time in between runs to do profitable missions — in essence, you'll be making money while you make money.