Where would I find out the best Uk Mobile Phone network provider in terms of coverage, tariffs, etc.?

I am currently with 3 mobile, and my contract is due to end in a couple of months. I hear that 3 mobile are in the process of improving their network, but are there any others that are much better than 3? I would like a smart phone, with unlimited internet and texts. What would be best network and/or deal for me?

http://www.allinsale.com smart phone offer , promotional mobile phone

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