Investors are bullish on marijuana stocks again, and now could be the time to buy some cheap pot stocks before they take off. With the Horizons Marijuana Life Sciences ETF down more than 70% over the past year, many pot stocks are trading at their lowest levels ever. And although some have been rallying in recent days, there are still some top cannabis stocks that are trading below $5 that investors may want to add to their portfolios today.

1. Aurora Cannabis

Aurora Cannabis (NYSE:ACB) finally cracked the $1 market again on Friday, when the stock popped by double digits. The stock's disappointed investors in the past with underwhelming earnings reports, and news that it was laying off 500 positions only exacerbated investor concerns about the company's future. Aurora's facing no shortage of challenges these days, and one of the most serious is its lack of cash flow. Investment banking company Ello Capital believes that Aurora may only have a couple of months' worth of cash flow to work with, and that was in February.

But Aurora still remains a top cannabis company in the industry. It generated 56 million Canadian dollars in net revenue in the second-quarter results it released in February. Through the first two quarters of 2018 and 2019, it brought in CA$131 million in sales. And in its 2019 fiscal year, Aurora's top line of CA$248 million was nearly quadruple its prior-year tally of CA$55 million.

The company's only anticipating modest growth, at best, during the third quarter. And with low expectations in place, there's potential for the stock to take off if it does well. There's new life in Aurora's stock of late, and this could be the start of a much bigger rally. A good performance in the third quarter could seal the deal and send the stock even higher.

2. Aphria

Aphria (NYSE:APHA) is arguably one of the safer pot stocks to buy. It's recorded a profit in two of its last three quarters and it's also generating much more revenue than Aurora. Aphria released its second-quarter results in January, and with more than CA$120 million in net revenue, that was nearly more than Aurora generated in two quarters combined.

But there's one reason that Aphria may not see as much of a bounce in its share price as Aurora: It hasn't fallen as hard. While Aphria's stock declined 70% in the past year, that's nothing compared to Aurora's 90% drop in price over the same period. However, with Aphria trading at around $3.50, it's well below its book value at a price-to-book multiple of 0.7 and a price-to-sales multiple of less than three. It may be one of the better bargains out there for cannabis investors. By comparison, Aurora trades at 0.5 times book value, but at six times its sales.

3. Curaleaf

Curaleaf Holdings (OTC:CURLF) was trading around $4 per share on Friday. The company's coming off the release of its year-end results on March 24, which pleasantly surprised investors as the company's revenue came in better than what analysts were expecting. Although it reported a loss of $26.6 million in the fourth quarter, its sales of $75.5 million were more than double the $32 million that Curaleaf recorded in the prior-year quarter. Analysts were only expecting revenue to reach $74.7 million in Q4. Curaleaf also reported pro forma revenue of $131.7 million, which includes pending acquisitions.

With a market cap of $1.9 billion and revenue of $221 million for 2019, investors are currently paying about 8.5 times sales for Curaleaf stock. It's also trading at five times its book value. However, given that its sales are larger than both Aurora's and Aphria's revenue numbers, and with a more lucrative U.S. market that it can tap into, investors may be more willing to pay a premium for Curaleaf shares. It's also been on a rough ride over the past year, losing more than half of its value during that time, and it could be overdue for a rally.

Which stock is the best buy today?

Despite its recent surge, Aurora Cannabis is still too big of a question mark to invest in today. From cash-flow issues to minimal growth and no profitability, there are many problems there that investors are better off not worrying about today. It's still a top cannabis stock, but it comes with some serious risks attached to it as well. Aphria is a safer buy given that it's at least proving to be more sustainable through its profits. But given the opportunities and the stronger sales numbers, Curaleaf looks to be the best cannabis stock of the three to buy today. And at a price of around $4, it could easily double with a few more good quarters under its belt.