Pay and go deals

Hi I want a new phone and can only get pay and go, have been shopping around for the best deals this is what I found:

o2 get 10% call time back


http://www.o2.co.uk/o2rewards10

Carphone warehouse top up £10 and get an extra £20 worth of calls

http://www.carphonewarehouse.com/commerce/servlet/gben-server-PageServer?ARTICLE=STATIC.FRESHNETWORK.FEB.SIMOFFER

Orange £5 free for every £10 spent

http://shop.orange.co.uk/shop/show/offer/freetexts

Any suggestions on which is best? Which ones do you guys use? Thanks