I think
Paddy Power and Bet
365 lead the way - I've had a couple of positive experiences with
Paddy Power and Bet
365 went to a lot of trouble to sort out problems with the site loading during an in-running game (even when the site was loading fine for everyone else) and stuck a £5 goodwill bet in my account as I'd missed out on the price I wanted.
I've had a bad experience with
Sportingbet not wanting to pay out on a promotion and I know from posts on this and other forums I'm not the only one.