Basically, an extension of your smartphone, a smartwatch has become a need for people who get a lot of notificaions on their phone throughout the day, and they find themselves distracted all day, having to pause to check the latest notification - taking out the ohone when it beeps / whirrs, looking, putting the phone back where it was. Repeat. All day long.
Obviously, in Apple's garden, mostly Apple products will sell.
Thus you get only Apple smartwatch.
Apple smartwatch offers the deepest integration with your iPhone of any smartwatch. There are lots and lots of apps that extend the utility of the smartwatch. The Apple smartwatch tracks all kinds of workouts. It looks elegant, true to Apple philosophy.
Although Apple smartwatch series 1 is now cheaper thna series 2 variant, series 2 does a getter tracking of your physical actitivity.
The best smartwatch for most iPhone owners: Apple Watch Series 1 (aluminum)
The best smartwatch for iPhone owners into water sports and fitness: Apple Watch Series 2 (aluminum)
The best smartwatch for iPhone owners who run a lot: Apple Watch Nike+ (same watch as the aluminum Series 2, but with Nike+ exclusive bands and watch faces optimized for running)