I think it depends on whether you text or talk. Vodafone have a good offer at the moment. Spend £2.50 monday to friday and get free texts all weekend, or spend £5.00 monday to friday get free calls and texts all weekend!! This coupled with stop the clock, pay for 3 minutes off peak and talk for up to 60, and the text pack offers I would say this is the best deal on the market at the moment.
o2 and orange both offer free texts with their online tarrifs, minimum monthly top up of £10 applies. T mobile have also launched some good value tariffs. Text appeal offers you 3p texts, 20ppm to landlines and other t mobiles, 40ppm to other networks, or they do a seperate tariff 12p calls to any network anytime, texts 10p.
You also have easymobile and fresh. 15p a minute any network, 5p texts