There's a huge variety of stuff.
For in-car use you really need something that will give you turn by turn directions. This is not as simple as it seems. You're heading north up the A3 and want to go west on the Hogs Back. Although the roads cross each other you can't get from one to the other in that direction, you have to take a link road half a mile before the junction. If you're heading east on the Hogs Back you can join the A3 northbound at the junction but not southbound. These are the things that a proper car navigation package will deal with. Memory Map will not, all it does is show your position on an OS map.
Everyone has their own favourite, you need to find out what yours is, the trouble is it's not easy to test drive them.