Over the last 15 years I've been on Orange, Vodafone, O2, 3, giffgaff, Lycamobile and just today I've had my number ported over to EE. As far as I'm concerned they have their good and not so good points.
Locally Vodafone & O2 are hard to beat in terms of indoor coverage while O2's 3G coverage has vastly improved over the past 3 years, with Vodafone still catching up.
EE's outdoor coverage is very good, though does suffer indoor more than O2 or Vodafone.
3's 3G coverage isn't quite as extensive as EE's but slightly better than O2 and well in front of Vodafone, however again in building coverage isn't good and there's no 2G fallback on Orange here any more.
Giffgaff is decent for what it is, just don't expect lightning data speeds via O2's network even through 4G. But for the majority of people it'll be fine and for calls & texts it's pretty decent. Lycamobile was OK at first but has well downhill recently with calls not connecting (despite the person called haaving full signal strength) and SMS not getting through not to mention recent reports of shady goings on apparently with them. So now I'm on EE - need a Signal Box from them to get indoor reception but I'll need to wait a bit before passing judgement with them. At present considering all things together, O2 just shades it for me.