I did fairly comperhensive testing off all networks and all around London, but it was 3 and a bit years ago now (before 4G and I think before EE?). This is data and not cell based.
Vodafone - great speeds if you could get a connection, rather unreliable
O2 - Good speeds, good availability (better inside buildings than others)
Three - amazing speeds, very good availability
Orange & T-mobile - poor speeds, poor availability
So I switched to Three. Then Three gradually got slower, their 4G roll out took forever. As that 2 year contract came to an end, I tried an EE SIM and was amazed by their speeds and coverage, so made the switch to them. Haven't had any problems, generally very good speed and availability.