For the sheer number of deliveries I've had, I have to say Royal Mail are the most consistent and my preference when purchasing. I've had some delays from time to time but I would say that the parcels seemed to be handled well from start to finish (e.g. I've had things arrive in good condition with minimal packaging, that wouldn't have survived with other carriers.)
Localised experience does make a big difference as others have said but I simply refuse to use Evri for sending and pray that the person has made the package bomb-proof, if they've sent something with them. DPD have been pretty good and they do seem most willing to provide you with infromation on a delivery window, which allows you a bit of freedom. The rest, being from NI, I wouldn't have a large enough sample size to really rate.