I don't think Willy Hill are that reliable these days at the risk of talking out of my arse but they seemed to be very slow paying out when I used them a couple of years back. Paddy Power are probably the fairest when its a dubious result. To be fair to Skybet they do good bonuses and stuff but they definitely ran out of cash on one event and took hours to payout, that was a few months back, might have been Epsom Derby but it was a big event anyway.
One thing I liked about BET365 is that they are very big on the security side of things to the extent of writing to you to send a code out, but then again some people might not like that when the missus opens the envelope for example! I never had any personal issue with Skybet apart from the one thing I noticed that didnt affect me, and if it is affiliated with Sky then in theory you would think they would never run out of money.
I have just put a tenner each way at 10/1 with Bet Fred. Its also tipped in the Racing Post so the odds may come down. But its a Handicap at Cheltenham and Nicholls and Walsh are on the favourite so i wouldnt back it to win.
I use 365, pretty easy to use, decent odds and they have a lot of sport you can watch on the streaming so of there's nothing on then you can watch a game from Argentina or some Italian lower league perfectly legally!