Fitbit Charge HR. Photo: Fitbit

Fitbit (NYSE:FIT) sells a half-dozen different wearables, including the inexpensive Fitbit Zip and high-end Fitbit Surge. But its single best tracker, and its best new product of 2015, is the Fitbit Charge HR. The device, released earlier this year, has become a major driver of Fitbit's business, garnering rave reviews and topping the list of best-selling fitness trackers, beating out many rivals, including Garmin's (NYSE:GRMN) suite of alternatives.

Dominating the middle
The Charge HR is Fitbit's second-most expensive tracker, but, retailing for around $130, straddles the middle of the wearables market. It's considerably less expensive than most smartwatches -- the least expensive Apple (NASDAQ:AAPL) Watch is $349 -- but more expensive than simple trackers like Xiaomi's $15 Mi Band.

Its functionality also lies somewhere in between. It can't run third-party apps or display most notifications. But it does offer reliable step tracking, sleep data, and a heart-rate monitor. It's not particularly stylish, but given its slim profile, it isn't conspicuous. It connects to Fitbit's highly regarded app, and syncs with virtually every sort of PC and mobile device available. Its battery lasts several days. For these reasons, reviewers have consistently recommended the Charge HR over its competitors.

"No heart-rate band costs this little, feels this small, and connects to as good an app ... It's the all-in-one fitness band I'd buy, especially for its low price ... none of the other watches and bands have been able to beat it at its own fitness game," wrote CNet's Scott Stein.

"The Charge HR is a good tool for monitoring daily activity and moderate workouts, and for getting better insight into what your body is doing....I would absolutely recommend it to my family and friends," wrote Wired's Josh Valcarcel.

"The Fitbit Charge HR is an excellent fitness tracker and continuous heart rate monitor, powered by the outstanding Fitbit website and mobile apps. For anyone looking to get into daily activity tracking, it's a wonderful device that measures things about your life that actually matter," wrote PCMag's Jill Duffy.

The Charge HR competes with a wide variety of trackers, but Garmin's Vivosmart HR may be the most direct alternative. It includes most of the same features, and is priced about the same as the Charge HR, but excels in one aspect: unlike the Charge HR, it's waterproof. Still, reviewers have continued to recommend the Charge HR over Garmin's tracker. Reviewing the Vivosmart HR, PCMag's Timothy Torres recommended that consumers "Get the Garmin band if you need something waterproof. But for everyone else, the Fitbit Charge HR remains a stronger choice."

Fitbit leads the competition
Fitbit has stayed atop IDC's wearable tracker chart in 2015. In the second quarter, it captured 24.3% of the market, edging out Apple, which captured 19.9%. Both firms saw their unit shipments rise in the third quarter, but their market share fall, down to 22.2% and 18.6%, respectively.

Fitbit reports how many devices it sells each quarter, but it doesn't break out individual models. Still, it's likely that the Charge HR -- which made its debut in January 2015 -- was a significant contributor to its 2015 dominance. In the third quarter, the Charge, Charge HR, and Surge accounted for 79% of FitBit's revenue, and research firm NPD Group reported that the Charge HR was the best-selling fitness tracker in the U.S. from April to September.

Fitbit shares rose on Monday after its app soared to the top of Apple's App Store chart. Presumably, those additional downloads were driven by new Fitbit owners unwrapping trackers -- possibly Charge HRs -- on Christmas.

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.