For me it's viewranger.
Easy to use.
East to follow on the lanes.
Easy to set up.
Cheap as you can use almost any current mobile phone handset.
Maps OS are about the same cost as other systems.
But you can use OS to plan route on the free MyViewranger web site.
You can also download free open source maps that work very well AND store free sat image maps on the phone for off line use (No mobile data needed).
You can also use foreign maps for trips abroad. I used it walking in Venice and driving on unsurfaced roads in Crete.
My "hand set" is my daughter old upgraded iPhone 3GS. Now about £30 unlocked on eBay.
I got a bespoke a 3GS water and shock proof case that mounts on the handlebars and a charging unit that connects to the bikes battery for £46 off eBay.
It's worked very well out on the lanes, OK in the rain, OK in the snow, survived numerous falls including one full emersion in stream crossing (phone case and handset, mount).
The phone also has a free normal sat nav app that runs at the same time as viewranger.
Phone has great camera and OK video AND will record video from the handlebars while VR is running. Reasonable quality...
Mount up clips in seconds so phone in the pocket when parked up etc..
Easy to share routes, upload routes, download routes and record where you have been.
It also has a neat buddy system where you can see where other users are on your phone in real time although you need mobile data for that so only good for unlimited phone data
tariffs.
I used to use Memory Map but got pissed off when they kept changing software and demanded you buy all the maps again. Also when their mobile phone app came out it simply didn't work and I didn't bother waiting for them to get it right so I got viewranger that worked immediately and was easy to use.
The tech help with ViewRanger is excellent and on the two occasions I needed ask questions the phone was answered immediately and proper simple advice given that worked immediately.
Planning routes on the my viewranger web site is very simple.
Saving the route and down/uploading/sync to the handset is quick and simple.
You can share routes with friends via the site. Very simple.
The use on the lanes is very simple, as is setting up what YOU want to see on the screen.
Personal view but I LIKE IT...