Depends on what you want. What you value. Chances are the better the CS the more the supplier has to pay to supply it, so the more they charge you for their service. You need to find a balance that suits you and how you use your phone.
I am using Tesco payg & as long as you top up before each month is up they give you extra credit, I also find that up to now their service is good too. They also use the O2 network which is good so who wants more ?
On the balance of things I would say O2, I have now been with them 7 years.
For what I require, they have been the best, competitively priced and good customer service.
OG: "not everyone thinks so". Obviously not....but I'd prefer the Which? report conclusions - based on the experience of thousands of their members- to the opinions of a few people on here..."I like A"..."I like B" etc. Statistically, so much more reliable.
Carphone Warehouse are really helpful as they deal with all providers. Signal strength is the main thing, they can do postcode checks for where you live / work to ensure you will be getting the most from your mobile phone. I'm with Talkmobile who use the same masts as Vodafone. Their contracts are cheap and customer service is great but if you go over your usage expect high charges! I wish I had gone for Giffgaff now though which is a sim only. You aren't tied into any contracts and can pick how much usage you want from month to month. You will have to pay for a phone up front if you go for this option though.
OG: and which provider am I "pushing"? As you can see from my first answer, I mention THREE providers recommended by Which?....not recommended by myself. So...hardly "pushing" a provider. Tesco Mobile is the ONE provider that is a Which? Recommended Provider for mobile phone service and my answer reflected this...since this is what the OP asked for.
Oh...and I seem to have the "Best Answer" too!