It is very location specific, around me Vodafone is 100% useless whereas at my parents (in the countryside) Vodafone is the only one you can get a signal with.
However, my wife's work phone is Vodafone and based on her experiences with that I wouldn't use them unless I had to (like my parents).
In my experience across all the different networks (we have all of them covered) I would say EE is best by quite a way, then Three, and then O2 and Voda distant last. Sister networks (e.g. GiffGaff, Virgin etc) seem to follow the network performance they are on, not surprisingly.