i used to be with orange, i quite liked em, customer service was always good and they replaced my phone free even when i was to blame when i broke it, they even paid my postage, (though i did say it just stopped working...
ah drunken wrestling, but thats another story

)
just recently moved to o2 though who have been pretty good, signal is a lil better on o2 for me, had to ring customer service once and they were great aswell