I have a Garmin Vivoactive 3 which is pretty good for tracking hikes, heart rate, hills, stairs etc, you can sync it to strava,my fitness pal and most other fitness apps. My brother has the Garmin Fenix5 Plus which has OS maps built in so works like a satnav for hiking, we used it yesterday on the Rhyd Du path and it beeped if you wandered off course, it was pretty impressive.
On my phone the best apps I have used so far are "Strava" and "All trails" both of them have premium features that have to be paid for to unlock the full functionality of the app but All trails has a lot of inbuilt hiking routes for your local area and you can overlay way point maps, weather reports etc, I pay 30 quid a year for it, but with that you get the OS maps.