The best deal for you will depend on lots of factors, particularly your consumption but also where you live, whether you want dual fuel, whether you want standing charges, whether you have Economy 7, whether you want an online tariff with paperless bills, whether you want to pay by DD and whether you want a fixed price deal and for how long.
The energy comparison sites are good provided you have pretty accurate info on your annual consumption.
You can also get cashback of maybe £30 if you switch through some comparison sites or via topcashback or quidco