While the price of copper is well below its all-time high, the metal's price could be on the upswing. That's due in large part to steadily growing demand, particularly from China, that's expected to more than keep pace with growing supply. As a result, copper producers see a bright future.

The best stock to invest in copper, in my opinion, is Freeport-McMoRan (NYSE:FCX) as its near-term copper-driven growth should pay off over the long term. However, in addition to Freeport-McMoRan, there are three other copper producers well positioned to capture future copper demand growth, making them great stocks to watch while investors wait to see if the copper story unfolds as the industry envisions.

1. Southern Copper (NYSE:SCCO)
Currently, Freeport-McMoRan is the largest publicly traded copper producer, which makes it the best pure play on copper. However, the company with the most copper reserves is actually Southern Copper. In fact, as the chart below notes, its reserves are almost double those of Freeport-McMoRan.

Source: Southern Copper Corp Investor Presentation. 

While Southern Copper controls vast copper reserves, its current production only ranks it fifth best. Not only that, but all of its production and reserves are in Mexico and Peru, which increases its geographic and political risks. All of that said, Southern Copper has the largest reserves and is currently is the lowest cost copper producer, which is what makes it such a key stock to watch in the copper sector.

2. Rio Tinto (NYSE:RIO)
Rio Tinto is actually just behind Southern Copper as the second lowest cost copper producer. This is due in large part to the company's world-class copper assets, which, as the following slide notes, are both large and low cost.

Source: Rio Tinto Investor Presentation.  

That said, the company's copper reserves are much smaller than either Freeport or Southern Copper. Further, the company is much more diversified outside of copper than those two, which is why it's not the best pure play investment in copper. Rio Tinto is better as a stock to watch in copper than the best stock to buy if the goal is to profit directly on growing copper demand.

3. BHP Billiton (NYSE:BHP)
Like Rio Tinto, BHP Billiton is a global resources company instead of a pure play on copper. That said, the company is one of the largest producers of copper and has an exceptionally large copper reserve base. Further, the bulk of the company's major projects are focused on growing its copper production, as is made clear on the following slide.

Source: BHP Billiton Investor Presentation. 

As that slide notes, BHP Billiton is investing 80% of its growth capital into petroleum or copper projects. Further, 100% of exploration capital is being invested in those two commodities. This is because copper expansion, like petroleum, offers the company returns in excess of 20%, which is due in large part to the strong projected demand for copper over the next decade. So, while BHP Billiton's future isn't entirely copper-driven, copper is a big future driver for the company.

Investor takeaway
The world is expected to need much more copper in the years ahead than is currently being produced. That's really great news for the world's top copper producers, as this demand will drive profitable growth for top-tier producers. While Freeport-McMoRan is best positioned to capture this future, Southern Copper, Rio Tinto, and BHP Billiton are also very well positioned to thrive should copper demand grow as expected.