What are your criteria for 'best'? I doubt they are exactly the same as others. That said, I don't rely on one app, preferring to use a general purpose navigation app, OSMAnd+, and complement that with a couple of specialist apps. This year I used
Wise Pilgrim and the Gronze app. I also used the Booking.com app on a couple of nights. And in addition, I carry a guidebook, normally one of
John Brierley's, but I have used others as well. Once I am close, I might use Google Maps for voice instructions if the place I am staying is a little off-route.
If pushed to choose only one app,
Wise Pilgrim would be my preference. It has good off-line mapping, for the CP it covers most of the variant routes for my walk from Valença this year, and covers most accommodation options as far as I could tell. More, it integrates well with WhatsApp. I only used the WhatsApp helper once, but it was a nicely implemented piece of functionality - a nice touch.