There´s no real way to know for sure which network offers the best coverage in your area, unless you have firsthand experience of using each of them. Some will obviously offer better coverage than others in certain areas and you can get a general idea of this from coverage maps. These maps will show you the typical level of signal strength for an area, from low to strong signal strength.
These maps are only for predicted coverage (i.e. a kind of guesstimate) and are not guaranteed. A coverage map may report poor coverage in an area that actually often has good reception for that service, and vice versa. In all places network coverage may fluctuate in quality depending on other conditions such as the weather.
There is no comparison of coverage between networks available, but you can find estimated network coverage data and coverage maps on each provider´s home page.
The information is presented in a slightly different manner for each network, with some easier to understand or more detailed than others. You may need to provide your postcode so that the page can display coverage levels in your local area.