OS Map Finder for me, for when riding on the South Downs, and (Hallwag) HKF-Maps for when riding in the Swiss Alps.
I'm not sure if I was just unlucky, but after recently upgrading to a new phone, I discovered that the OS map tiles couldn't be transferred to the new phone. I tried and the shop tried, but it wouldn't let me transfer them across. In the end, it didn't prove to be an issue, as I was able to continue to use the old phone for mapping, with no phone contract required.
I also once discovered that it pays to buy the map tiles in advance of where you might be riding. I wanted to buy a map tile when out on a ride, and the system wouldn't allow me to do it.
HKF-Maps operate a really good system, where by you buy a paper copy (which I like) then just use a download code to install the map onto your device. HKF-Maps, offer specific mapping for both mtb and road cycling, along with suggested riding routes, which amount to an endless selection of riding route fun. The system also provides ride length,(if sticking to set route) elevation gain, degree of route difficulty on a scale, level of fitness required, predicted ride time, info of any obstacles or hazards, along with advice, and even points of interest in respect of taking photos.
In reality my OS mapping doesn't get much use, as I tend to just ride off in no set direction, route or plan in mind. I prefer to just head out and see where I end up, often just following trails that animals have made.
Living in the South East, you might occasionally become lost, just as I did just the other night, but you are never going to be that far from a road etc. In about 12k of off road riding, I have only been lost or rather disoriented twice. Once in the fog on the South Downs during a night ride, and then last week during another night ride.