Where they are still under some form of effective state control - Merseyrail, the London Overground, East Coast (before it was sold off to Virgin - they are of quite a high standard. There are also one or two good private companies running services on part of the network - Chiltern (who run the route out of Marylebone to Birmingham) are great, and the Wrexham and Shropshire Railway (which received no subsidy at all) was the best service in the country before unfair competition from Virgin killed it off.
As for the rest, in my experience Virgin are terrible, South West Trains and Southern tolerable at best, Great Northern alright.