This depends greatly on how each person wants to use it...
The Suunto watches have the reputation for highly rugged, go anywhere uses, like ultra runners, rugged cross country, mountain biking, hiking, etc. You can beat the crap out of them, and the batteries last a very long time. A friend of mine races 3 and 6 hour mountain bike races. He wears a Suunto 24/7.
Garmins are OK at activity tracking (good enough, but not as good as Fitbit), and second-to-none at fitness. Runners and triathletes gravitate toward Garmin. Garmin also has some very solid crossover devices, like the Vivoactive and Vivofit that do both fitness and activity, with more of an emphasis on the activity tracking.
In my humble opinion, the Apple Watch is the worst in market in both fitness tracking and activity tracking. It is not that the hardware is lacking-- the AW device can do most of the same things as Fitbit, Garmin, or Suunto. It is that the AW has very limited software support for fitness and for activity.
Apple's strength is the user interface, its appearance (though, a lot of people love the Fenix 3 and a bunch of the Suuntos on looks), apps, the future potential when more apps arrive, Siri, Apple Pay, etc.
My guess is that someone buying a Suunto or Garmin prioritizes activity tracking and fitness tracking in their lifestyle. An AW buyer is likely more interested in the technology and looks. Personally, I use a Garmin for fitness, Fitbit for activity (though I may be shifting to Garmin), and AW for the cool tech stuff.