As we only reccommend the best networks to try for a sim only deal there are 4 choices if you want to get the best chance of a mobile contract:
Vodafone Sim Only Contracts - Vodafone has its "super sim" deal which is well, super really. It is the cheapest and in our opinion offers a great deal more with Vodafones great current promotions.
T Mobile Sim Only Contracts - T Mobile has the best acceptance rate for SOLO tariffs and so they are an ideal choice when looking for a bad credit mobile contract.
O2 Sim Only Contracts - O2 has a similar pricing structure to Vodafone and also offers perks such as free o2-o2 calls and free mobile internet on some of the higher sim only tariffs.
Orange Sim Only Contacts - Orange is not quite as competitive as the other networks but it does have a good acceptance rate if you do want to upgrade at a later date.