That depends on your circumstances, stick the details in one of those comparison sites and see what it comes up with. That's the best place to start.
Mine's through First Utility, may not be the cheapest now but their tarrif is one straightforward price, I can read my meters any time and work out exactly how much it's costing me on any particular day. I've even managed to construct a spreadsheet which makes the calculations when I put the meter readings in each day. Yes I'm a sad so-and-so! Wondered how much I actually used a couple of years ago when I was left on my own, needed to make some cuts and found that this meant I can read my meter and calculate my usage exactly.
They also give £284 of dual fuel discounts in a year. I'm alone, working full time (35 hour week) and feel the cold, last year my total spend was £891.77 after taking the discounts off. Got a free month-and-a-half in the summer because of them. I've made a provision of £100 a month for gas & electric since July 2010 and I've now got £320.98 lying in my bills account for 'energy'.