Maps for cycling
The best scale for cycling is 1:100 000. Unfortunately there aren't many of these around since the demise of the Barts map series. So we are left with the Ordnance survey maps.
The landranger 1:50 000 maps are best (in terms of detail and accuracy) but are only really suitable for short 1/2 day rides because you will soon ride off the edge. You should have the 3 or 4 that cover your local area.
For longer rides you need the next size up which is 1:250 000. Either buy the OS travel master maps or buy a cheap AA road map and tear out the pages.
August 05 edit: Make sure your maps are up to date! Recently I was about to go out for a ride and borrowed a map. As I would be cycling I was about to take the older version, rather than damage the newly bought version, and was just flicking through the route when I saw that something was wrong. I couldn't work out what it was and spent quite a while trying to find what had rung an alarm bell in my mind. Finally I got the new map out to compare it, and found that the map I was about to cycle with had the M25 missing! I would have had quite a major problem following my route! End edit.
The advantage of the larger scale OS maps is that they also include details such as contour lines, helping you to plan a less hilly route. The road atlases don't have this information, although some do still mark the steepest hills with a small arrow across the road.
Some maps have “cycle routes” marked on them. These can be helpful in planning your journey, but you have to be aware that in their eagerness to provide a traffic free route the route planners sometimes take to bridleways and other loosely surfaced tracks which may not be as suitable for cycling as you would hope. Some of these routes are also renowned for taking a much longer distance than a route on suitable quiet roads. In towns these routes can take to many minor roads and pavements with many junctions, again this can slow your journey down dramatically. You need to plan for this if you are on a time schedule!
I was planning on putting some photos here to demonstrate the different levels of detail that are available on maps, but Ordnance Survey want £50 odd pounds off me per year for the privilidge of reproducing and advertising their wares. As a voluntarily run website I just can't afford that. Sorry.