IIRC first went with O2 sometime in the 90s as at the time they were about the only one that had an overall presentation of a professional company working to a standard and had their head in the game - sadly not something they stand out for any more. Around 2007 I swapped to Vodafone and dropped O2 (though kept a payg plan as a backup until recently) sometime around 2009 as Vodafone had better offerings, better prices and more forward looking in approach and atleast around the areas I frequented while O2 you had a bit better chance of getting any kind of connection at all in remote parts anywhere you had a signal from both providers Vodafone had better call sound quality/less dropouts and better data speeds. I've stuck with Vodafone for the last ~12 years for the most part they've just worked and offerings have been reasonable though I'm getting somewhat annoyed by their constant advertising - some weeks I get like 3 letters, 5 text messages/calls and 1-2 emails advertising for their ****** broadband and if I was ever going to take it up I certainly won't now! I've recently started using Virgin Mobile (who use EE) for a secondary provider and 4G data and so far been fairly impressed to be fair.
Along the way I've used other providers like GiffGaff mostly for secondary/backup or failover data purposes, etc. but largely the results haven't been great and family members who use or used some of the other providers have had a worse experience over the years than I have.
Hmm, I will likely stay with Virgin Mobile tbh, the data package is really good with rollover and the price matches Three (or beats even). Might switch to EE depending on what the 5G uptake is like in a year's time or two.
I have to say I really like Virgin Mobile's approach to data rollover - I'm not sure if it is intentional or an oversight on their part as it usually works out quite generous - more so than other providers - and a big factor in me potentially sticking with them - it has been a godsend for me after moving to a place with poor wired broadband but able to somehow get a decent 4G connection with VM/EE despite conditions that you wouldn't think would enable that.