RE: Best app for Field Service Technicians
You can either use F1 Sky App (Field Service Mobile) or RESCO's app
You will have limitations of 10 entities I think you can use free of cost if you use RESCO and will need to pay if you want to use more than that for custom entities however you get more functionality with that as compared to F1 app.
We have used F1 app/Field Service Mobile app for couple of clients and had no issue, and also used offline HTML to run JavaScript and use it for validation purpose etc.
In one scenario engineers have Samsung devices (android) and using app and no complains.