The U.S. State Department’s inspector general recently issued a report that dealt another blow to those fighting against the Keystone XL pipeline. The report concluded that there were no violations of conflict of interest rules when Environmental Resources Management was brought on to conduct the environmental assessment.

Many people believe this will help pave the way to an approval from President Obama. But what’s the best way to bet on that happening? Oddly enough, it’s probably not buying TransCanada (TSX:TRP)(NYSE:TRP) shares. The $5.3 billion pipeline represents only 14% of the company’s commercially secured projects. Even if the company earns a 25% return on investment from Keystone, that adds less than $2 per share in value (the shares currently trade at about $50).

It’s actually the Canadian energy companies that would benefit most from an approval, and three are worth highlighting.

Canadian Natural Resources

Whenever Canadian Natural Resources (TSX:CNQ)(NYSE:CNQ) presents to analysts & investors, it seems like a majority of the time is spent talking about heavy oil differentials. The company produces nearly 94% of its oil from Canada, and has already committed to moving 120,000 barrels per day of crude on Keystone. Canadian National does not own any oil refineries or gas stations, and thus is not hedged against long-term price differentials.

One analyst predicted that the stock price would jump 5% if Keystone is approved and drop 3% if it is rejected.

MEG Energy

MEG Energy (TSX:MEG) has been referred to as the “go-to, pure play, oil sands stock”. And it’s easy to see why. The company’s entire production comes from heavy oil operations in Alberta, and like Canadian National, MEG has no refining or retailing operations.

Perhaps more than any other oil sands producer, MEG has hedged its bets with regards to Keystone. The company has invested heavily in rail-loading facilities and should be able to transport oil to the gulf coast for only $15 per barrel by using both rail and barge.

But MEG still stands to benefit immensely from Keystone’s approval. The current shortage of pipeline capacity has also caused a shortage of rail capacity, a trend that will likely get worse as oil sands output soars over the next decade. Keystone’s approval would help to mitigate that.

Blackpearl Resources

Blackpearl Resources (TSX:PXX) may be more reliant on the Keystone decision than any of its larger peers. Like MEG, 100% of Blackpearl’s production comes from Western Canada. But unlike MEG, Blackpearl is short of funding for most of its expansion projects. The company had to split one of its projects (Onion Lake thermal) into two phases, and has deferred its larger project (Blackrod) until a joint-venture partner can be found. But given today’s operating environment, joint-venture partners can be hard to find.

So not only would Keystone’s approval allow Blackpearl to get better pricing for its product, it may give the JV market a boost as well, making it easier for Blackpearl to fund its ambitious growth plans.

Foolish bottom line

Keystone’s approval is still far from a foregone conclusion, so making an especially large bet would be very risky. But anyone who’s confident in Keystone’s fate may want to consider one of the names above.