The latest State of Mobile Networks: UK report from OpenSignal has EE as the clear leader according to its metrics, with O2 and Three needing to raise their game.

You can see the charts for each of the nationwide 3G and 4G metrics below, but to cut a long story short EE won nearly all of them. It also leads in 4G availability with 86.6%, followed by O2 on 83.4%, Vodafone on 79.5% and Three on 66.6%. However our average 4G speed of 23.1 Mbps apparently lags even Armenia and Mexico. Maybe the extra 4G spectrum O2 just got hold of will help it do better in 4G next time.