If you are walking in Great Britain then there are three obvious choices for the range of maps to use, and two (Active Maps) which you may also want to consider.
Ordnance Survey Landranger, 1:50K scale maps, cover the whole of Great Britain in 204 single sided sheets ranging from Sheet 1 Shetland - Yell, Unst and Fetlar to Sheet 204 Truro & Falmouth, Roseland Peninsula.
Click on the sheet you are looking for in the map on the right to see the coverage of that sheet.
A small number of sheets in popular walking areas like the Lake District and Snowdonia are also available as "Active Maps". These are the same as the standard maps but laminated to make them waterproof. They are typically almost twice the price and are bulky and we find it easier to stick with un-laminated maps and a good map case.
Ordnance Survey Landranger, 1:25K scale maps, cover Great Britain on 403 sheets, some of which are double sided which is a mixed blessing.
For historical reasons the numbering scheme is a little confused. Sheets below 100 are prefixed by "OL" and form part of the Ordnance Survey "Outdoor Leisure" range. Like Ordnance Survey Landranger Active maps they cover popular walking areas.
The rest of the UK is covered by maps running south to north starting with sheet 101 Isles of Scilly to 470 Shetland - Unst, Yell & Fetlar.
All explorer of sheets are also available as "Active Maps". These are the same as the standard maps but laminated to make them waterproof. As with the Landranger Active maps they are typically almost twice the price and are bulky and we find it easier to stick with un-laminated maps and a good map case.
Harvey offer their own range of walking maps published in co-operation with the British Mountaineering Council. Some are at a scale of 1:25K, others at 1:40K (the map on the right shows their 1:25K range).
Harvey maps are specifically designed for walkers and are made of polyethylene, not paper. These maps are tough, durable and 100% waterproof. They are also compact - not the great bulky package you get with a laminated Active Map - and light to carry.