On the one hand, marijuana is supposed to be the greatest growth opportunity since sliced bread. Worldwide sales of cannabis have more than tripled over the past four years, and they could grow anywhere from fivefold to 18-fold over the next decade, depending on Wall Street's projections.

On the other hand, the industry has been a train wreck for investors since the end of March. The Horizons Marijuana Life Sciences ETF, the first exchange-traded fund focused on cannabis, has shed 40% of its value in a six-month period. And that's including the dividends the ETF has paid out.

Cannabis supply issues in Canada, high tax rates in select U.S. states that have given rise to the black market, poor operating results, and ongoing share-based dilution have been a four-headed monster that's sent a number of pot stocks up in smoke.

While it's quite evident that the industry has some growing up to do, there are a small handful of marijuana stocks that look like genuine long-term bargains. Here are the top marijuana stocks you should consider gobbling up during the fourth quarter.

A handful of cannabis buds lying atop a messy pile of cash.

Image source: Getty Images.

Planet 13 Holdings

Vertically integrated dispensary operator Planet 13 Holdings (OTC:PLNHF) is easily one of the most exciting cannabis stocks as we enter the fourth quarter. Whereas most pot stocks have struggled with transparency, as well as standing out in an increasingly crowded field, Planet 13 manages to check off both boxes.

The operator of the largest retail dispensary in the world, just west of the Las Vegas Strip, Planet 13 publishes data monthly on foot traffic, average check size, and paying customers. Since opening its SuperStore last November, average daily foot traffic has more than doubled (through August), with the average ticket increasing by more than $11. Planet 13 is also responsible for about 10% of the state of Nevada's total weed sales. This is impressive considering that Nevada might be the most lucrative market in the United States, by some accounts. 

Planet 13 also offers something unique: a 112,000-square-foot SuperStore. Soon it'll also offer a 40,000-square-foot location in Santa Ana, California, just minutes from Disneyland. Planet 13's dispensaries are must-see experiences for cannabis enthusiasts, with unparalleled product diversity, plenty of interwoven technology, and lots of product immersion.

There's simply nothing like Planet 13 in the marijuana space. With the company possibly on track for full-year profits in 2020, it looks to be worth considering as a buy.

A cannabis leaf laid within the outline of the Canadian flag's red maple leaf, with rolled joints and a cannabis bud to the left of the flag.

Image source: Getty Images.

OrganiGram Holdings

Atlantic-based grower OrganiGram Holdings (NASDAQ:OGI) holds the unique distinction of being the only company former Canopy Growth co-CEO Bruce Linton wasn't affiliated with that he would suggest investors buy. That's pretty high praise (pardon the pun) from one of marijuana's most prominent visionaries.

One thing that makes OrganiGram so unique is its growing location. It's the only major grower -- i.e., Canadian producer expecting to yield more than 100,000 kilos at its peak -- located in an eastern Atlantic province. The company's Moncton, New Brunswick, location should produce 113,000 kilos annually when fully operational, and will have clear geographic advantages in serving eastern Canadian cannabis users. Though eastern Canada is less populated, the percentage of pot users in these provinces is higher than the national average.

Additionally, OrganiGram has some of the best yields in the entire Canadian pot industry. Management has stuck by its 113,000-kilo annual output, which would place its yield at 230 grams per square foot. Most of its peers are producing between 75 grams per square foot and 125 grams per square foot. OrganiGram's proprietary three-tiered growing system can be thanked for its superior efficiency.

With derivative sales set to kick off in Canada by mid-December and OrganiGram ready to launch a line of chocolate edibles, a nano-emulsification technology in powder form for beverages, and vape concentrates, it looks as if it'll be among the first Canadian growers to become profitable on a recurring basis.

A vial of cannabinoid-rich liquid lying atop cannabis flowers.

Image source: Getty Images.

MediPharm Labs

If it hasn't been mentioned enough, derivatives are going to play a major role going forward for cannabis stocks. That's because derivatives, such as edibles, vapes, beverages, topicals, and concentrates, offer much better margins than traditional dried cannabis. And that's where extraction-services provider MediPharm Labs (OTC:MEDIF) comes into play.

There's a simplistic beauty in MediPharm's business. It locks up contracts for one or more years to process hemp and/or cannabis biomass for targeted cannabinoids or concentrates. It can then deliver these extracted goods to its contracted partner or work as a white-label producer of specific products, such as cannabidiol oil. In other words, the rise of derivatives makes MediPharm Labs an indispensable middleman that'll likely generate very predictable cash flow and profits from its intermediate-term contracts.

Although MediPharm has signed up a number of growers, its most notable deal involves providing $30 million worth of cannabis concentrates to Cronos Group over an 18-month period. The deal could be doubled to $60 million over two years, depending on demand. Further, Cronos is utilizing MediPharm as its contract manufacturer for its COVE line of vape products.

Maybe most impressive is that, despite beginning extraction work last November, MediPharm already turned an operating profit in the second quarter of 2019. With extraction capacity eventually on its way to 500,000 kilos per year, imagine what'll be possible for MediPharm Labs.

An up-close view of a premium-quality flowering cannabis plant growing indoors.

Image source: Getty Images.

Flowr Corp.

A fourth and final top marijuana stock to buy in the fourth quarter is Canadian grower Flowr Corp. (OTC:FLWPF). Flowr recently issued stock to help with its acquisition of Holigen, and passed on its approval to uplist to the Nasdaq exchange. As a result, its stock has been clobbered, along with its peers. But this pain could be investors' gain.

As I've emphasized throughout, uniqueness is of paramount importance in the highly competitive cannabis industry, and that's exactly what Flowr brings to the table. Unlike its peers, Flowr focuses on producing premium and ultra-premium dried flower and derivatives. There's very little competition in the premium-quality product space, which means virtually no pricing competition and hefty margins.

Just as important, Flowr is working with Scotts Miracle-Gro subsidiary Hawthorne Gardening to boost productivity at the Kelowna campus in British Columbia. Flowr has already guided toward 300 grams of yield per square foot in 2019, with the duo working toward 400 grams of yield per square foot by 2022. This production efficiency is unsurpassed.

There's also the aforementioned acquisition of Holigen, which will bring a 7-million-square-foot outdoor grow farm into the fold. When fully operational, this Portugal-located Aljustrel farm could produce 500,000 kilos per year for the burgeoning European medical pot industry. When combined with Kelowna's 50,000 kilos of premium grow and the adjacent Flowr Forest, this company is on the verge of becoming something special.