The most useful features for me in descending order.
1. Notifications and Glances - no need to take out phone to look.
2. Calendars and Appointments - to remind me my next activity
3. Fitness & Health tracking - closing the activity rings has become an obsession.
I've had my Apple Watch Series 1 for 2+ years now.
There is one thing about the Apple Watch that I'm really disappointed with and that's the Heart Rate Monitor. The initial measurement is always wrong! Granted it will settle to an accurate measurement after a while but the initial HR measurement is the one being logged into the HR chart. For example when I fire up my HR app, my initial measurement will be something like 80+bpm before settling down to the correct 40+ or 50+ bpm if I'm just sitting round.
So my HR log taken automatically every 10-minute interval is highly spurious and inaccurate. A check with the log shows something like 80, 100, 90, 60, 45, 90 within the hour. My HR rarely goes above 60 when I'm just sitting in office doing work. Even when I'm walking it is around 55-65 bpm. I need an accurate resting heart rate data to give an indication if I have recovered from the previous day's run. I do a lot of distance running and resting heart rate is one metric that I track, and the Apple Watch failed in this department.
I've returned and gotten a replacement Apple Watch before and it behaved the same. I've also tried my friends' Apple Watches and it's the same story.
If Apple Watch Series 3 HR can obtain the initial HR measurement accurately I'd buy that in an instance.