Sleep++ is great if you don't mind wearing your watch to bed. The app will keep a log of your sleep pattern throughout the night and provide you with a concise analysis of your light sleep, deep sleep, restlessness and wakefulness.
This is one of the few third-party apps I keep on my Apple Watch. Reminds you to drink water in accordance with a target you set. Also has a ring complication that you can have alongside your other rings (activity and battery)!
I prefer Pedometer++ over the default step counter in iOS. Although I can't or haven't proved it, the app seems to be more accurate with its step count compared to the system count. Setting daily step goals with the app is also a nice bonus.
This is another app I love and rely on daily for info regarding my progress with steps, calories, and heart rate. It's from @crunchybagel, makers of Streaks and Streaks Workout. Both fantastic apps in their own right.