F-Series pickups, especially loaded ones like this 2016 F-150 Limited, are big contributors to Ford's bottom line. Image source: Ford Motor Company.

What was Ford Motor Company's (F -1.57%) "best vehicle" in 2015?

That's easy, because it's the same answer every year: The F-Series pickup family. But 2015 was an especially interesting year for America's best-selling vehicle line.

What makes the F-Series Ford's "best vehicle"
For investors, this isn't about zero-to-60 times, or fuel efficiency, or towing capacity. It's about profits. The exact details of any vehicle's profitability are a closely guarded secret, but it's a safe bet that few vehicles anywhere generate the kind of profits Ford's big pickups bring home.

At first glance, 2015 looks like kind of a blah year for the F-Series, with U.S. sales up just 2.3% through November. But that's deceiving: They're up over good 2014 numbers despite months of shortages of Ford's super-popular F-150. And make no mistake, that's a lot of trucks: Last year, Ford sold 753,851 F-Series trucks in the U.S., making it by far America's best-seller. It's on pace to beat that number by a few thousand in 2015.

It's not just Ford's best-seller, it's also very profitable
But it's not just the F-Series sales volumes that make it such a big deal. Each of those trucks brings home a good chunk of profit, on average. Consider this: Ford's F-150 starts at just over $26,000, and its Super Duty siblings at a little over $32,000. But almost nobody buys a bare-bones pickup, especially from Ford: The average transaction price for an F-Series truck in November was $42,800. 

Ford's Super Duty trucks will get some of the new F-150's magic next year. The 2017 F-Series Super Duty features aluminum body panels, along with a seriously beefed-up steel frame. Image source: Ford Motor Company.

By the way, that average transaction price is up about $2,700 from a year ago, Ford officials say. Low interest rates, cheap gas, and longer-term loans have probably all played a role in that increase. But Ford's feature-laden all-new 2015 F-150 should get a lot of the credit, too.

Many Ford trucks are also luxury vehicles, and priced accordingly
There's a lot of profit in the difference between a base-model pickup and a loaded one, and Ford is very, very good at enticing buyers to pay a little more for features that were unheard-of in a pickup truck 25 years ago. Ford essentially invented the idea of a luxury pickup with its first King Ranch model back in 2001.

Rivals General Motors (GM 0.40%) and Fiat Chrysler (FCAU) have since followed suit, with luxury pickups of their own, but Ford still holds the sales lead. The F-Series isn't just America's best-selling vehicle, it's America's best-seller over $50,000, too. 

The 2001 Ford F-150 King Ranch was the first of Ford's luxury pickups. Many more have followed, and they've given the Blue Oval's bottom line a big boost in recent years. Image source: Ford Motor Company.

Long story short, given its huge sales volumes and likely profit margins, the F-Series is far and away the biggest driver of profits for Ford in North America -- and North America is far and away Ford's most profitable market region.