We often here which mobile phone network had the overall best coverage for various types of service from many sources and many people, but in the end it's a highly personal thing that depends on many personal factors and usage requirements, no one person can say to another with any confidence which is best for someone, and service levels and quality have changed so much in the past few years anyone saying service or coverage was bad when they tried it a few years ago might find things very different now or in the next few months.
I changed from three last year who were the best network in my locations to Vodafone, who if you look at coverage maps are a solid 3rd place for coverage or any kind, in Shropshire and South Cheshire behind O2 & EE,
But for me I had a full usable 2g signal indoors and 4g upstairs at home last year, 3g full signal at work and 3g 90% albeit slow 900mhz on my commute with Vodafone vs threes no indoor coverage, very weak 3g with no throughput on my commute and a congested non responsive 3g & 4g at work, it was a no brainer really as those are the areas I use my phone most of the time. Now, Vodafone have improved things that I now have 3g/4g indoor coverage at home, on most of my commute and at work I have had 4g since the next year, with at least 4 more mast I pass coming online in the next few weeks..
But if you live in Devon, hants, big parts of Wales and wast anglea, you will probably still think them the devil incarnate and rightly so.
Myself and other think the same about EE where I live as they have not upgraded nearly all the orange 2g masts and left the Mbnl masts on 2mb copper backhaul in the most part.