I think a lot depnds on how much you use your phone and when. If you use your phone regularly at peak times then probably T Mobile, Tesco and Asda are good.
If, like me, you only make very infrequent brief calls during peak time, and most of your calls are off peak then Vodafone Anynet with stop the clock is very good. After 7.30 pm and at weekends you only pay 10p per minute for the first 3 minutes and then its free for up to 1 hour. thwerefore 30p per hour offpeak! However, at 30p per minute at peak times this is not so good for peak time calls.