The brand leader is Tom Tom - I have one and have found it to be excellent being easy to operate, clear in its instruction, and reliable in location and directions.
I believe it is among the less expensive for a basic model, as with all things, you get what you pay for, and the more sophisticated models have additional features - it depends how often you use it, and how many of the features you will need and use.
I got a navman off ebay. It was new and from a seller who sells nothing but sat navs. Very simple to use as you can just take it out of the box, switch on and you are off. My husband has to charge his Tomtom the night before. I got it for him, second hand reconditioned off Amazon.