when choosing suppliers I use:
www.uswitch.com. type in your details and it tells you the cheapest, or the greenest, or the friendliest supplier in your area. can be used for phones, internet, teley, gas, electric, water etc.
as for personal experience. i've used powergen for my gas/electric and anglian water for my duh! water. been reasibily happy with both. however, i haven't had a particular problem that needed solving so as for customer service I can't vouch.