Paper OS map from WHSmiths in one of their 2 for 1, 3 for 2 etc deals. Far cheaper for 25k maps, and they don’t run out of battery 😛
Seriously though, if it’s somewhere I really need a map, that’s what I do, then scan the bit I want, print a copy and stick it in a plastic pocket thing in the bag. Or you could scan and stick it on the phone. I don’t navigate with the stuff, just use a map to look at the area in advance or odd occasion get out on a ride to get my bearings.
But as for the apps, ViewRanger is probably the best one. The OS maps are fine, just overpriced for 25k in my opinion. I use OpenStreetMap stuff on it instead which is free, contains all the key information really plus adds contributed trails in some locations you won’t find on an OS map.