Giff Gaff / O2 - have I understood folk traveling and adventuring away from the main lines of communication in Scotland, the GG/O2 is currently the best for connectivity? I had nowt on the Corrour estate when covering a Silver DofE whilst my colleagues on Giff Gaff had text reception at least.
They have agreements to share a lot of equipment and infrastructure so there are similarities in coverage between EE/H3G and O2/VF. A lot of sites are moving/have moved away from shared to unilateral equipment (with some splitting to completely separate sites) so the levels of coverage are diverging further all of the time.
The different networks also operate different frequencies which leads to more divergence.