Bet365 are very good. Gave me a £5 free bet once as I couldnt make a deposit in time to place a bet as there site was playing up.
Paddy Power are also excellent and very fast.
I know its not one of those mentioned but Olympic Sports are terrible.
I had to email them a dozen times to get an answer, it was more like being in a chat room with them and there one line replies.
I ended up getting an email from the boss explaining my query in detail after the muppet I was speaking too didnt have a clue.
SkyBetVegas was poor aswell. I emailed them about a login problem I have but they didnt ever reply.