There is no definitive answer to who is the best and worst energy supplier in the UK. The only way to measure such a thing is to go by surveys from independent organisations, look at customer reviews and look at data from the regulator, Ofgem.
However, using this data you can see that some energy companies frequently score well and others generally prop up the tables. So Energy is usually near the top of the pile, scoring consistently well for value and customer satisfaction. Good news if you are thinking about switching.