2.2 billion gamers will generate $108.9 billion in global gaming revenues this year, according to research firm Newzoo. That's a 7.8% increase from 2016.

The U.S. is the second largest gaming market in the world after China, and is expected to generate $25.1 billion in gaming revenues for the year. PwC estimates that the U.S. video game market will grow at a compound annual growth rate (CAGR) of 3.6% between 2015 and 2020.

A couple plays a video game together.

Image source: Getty Images.

In the U.S., two gaming genres topped all others throughout the year: shooters and sports games. Let's take a closer look at the five top-selling games of the year, according to research firm NPD Group.

1. Call of Duty: WWII

Activision Blizzard's (NASDAQ:ATVI) Call of Duty: WWII is the 15th main series title in the 14-year-old franchise. It also marks the series' first World War II game since Call of Duty: World at War in 2008.

Call of Duty: WWII.

Image source: Activision.

The game was launched for the PS4, Xbox One, and Windows PCs in early November, and was met with positive reviews. Gamespot rated it a 9 out of 10, lauding it as "one of the most comprehensive and filler-free Call of Dutys in recent memory" which "successfully capitalizes on the series' strengths."

As of Nov. 4, 2.28 million copies of the game had been sold across the three platforms in North America, according to Vgchartz -- a figure which will likely keep rising as the year ends.

2. Destiny 2

For many years, Activision bears feared that the publisher couldn't follow up its hit Call of Duty series with another big shooter franchise. But Activision proved them wrong with 2014's Destiny, an online first-person shooter from Bungie, the makers of Halo.

In September, Activision launched Destiny 2 for the PS4 and the Xbox One, and for Windows PC the following month. 2.04 million copies have been sold across all three platforms in North America.

Destiny 2.

Image source: Activision.

Destiny 2 received higher reviews than its predecessor. Gamespot, which rated the original 6/10, rated the sequel an 8/10, calling it "a significant improvement over the original and a better experience for more than just the most hardcore players."

3. NBA 2K18

Take-Two Interactive's (NASDAQ:TTWO) NBA 2K18 is the latest annual installment of the NBA 2K series, which started in 1999. The game was launched in September for the Xbox 360, Xbox One, PS3, PS4, Nintendo Switch, and Windows PC, and sold about 1.8 million copies in North America so far.

The NBA2K series attempts to emulate a "real" NBA experience, with up-to-date team rosters as well as acclaimed teams from previous eras, like the 1995-1996 Chicago Bulls and the 1985-1986 Boston Celtics. Kobe Bryant and Kevin Garnett also appear as guest commentators in the latest game.

NBA 2K18.

Image source: 2K Games.

The game was generally well-received, but Polygon, which rated the game a 7/10, criticized its use of microtransactions, which it claims hit "a new peak" with NBA 2K18.

4. Madden NFL 18

Electronic Arts' (NASDAQ:EA) Madden NFL 18 continued to dominate the football genre with the series' 29th main installment. The game was released on the PS4 and Xbox One in August, and sold about 1.71 million copies in North America.

The main changes to this year's installment, beyond the rosters, include a shift to EA's Frostbite engine and a new "Longshot" story mode in which the player follows the career of an underdog NFL draftee.

Madden NFL 18.

Image source: EA Sports.

Electronic Gaming Monthly gave the game a 7/10 rating, praising the "Longshot" mode but stating that it "misses the mark with a few of its gameplay additions this year -- so if you don't immediately take a liking to them and choose to ignore them, the experience will feel a lot like last year's."

5. Tom Clancy's Ghost Recon Wildlands

Ubisoft's (OTC:UBSFF) Tom Clancy's Ghost Recon series is a 16-year-old tactical shooting franchise, and the latest entry -- Wildlands -- is its first open world game.

Tom Clancy's Ghost Recon Wildlands.

Image source: Ubisoft.

The game was released in March for the PS4, Xbox One, and Windows PC, and went on to sell 1.64 million copies in North America. It received mixed to positive reviews.

Gamespot, which rated it 7/10, stated that it "stays true to the series' Rainbow Six-inspired roots," but that it was also a "middlingly safe tactical shooter and a slightly wasted opportunity given the ambitious scope of its seemingly boundless map."

The key takeaways

The top selling games of 2017 generally teach us two things: Shooters and sports games still dominate American gaming, and established franchises still beat new IPs. This is a blessing and a curse for publishers, which generate steady revenue from these series but become increasingly cautious about launching new IPs.

This trend will likely continue over the next few years, with the number of installments in aging franchises potentially outnumbering the number of brand new series.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.