When it comes to an organisation looking after your money, I think you really have to be 99-100% sure that they are legitimate. Top sites with good reputations include all the "biggies" like Party, Poker Stars, Ultimate, FTP, etc. It's best to steer clear of small, less established sites asking you to send highly confidential info in my opinion (this is why I didn't touch Mansion Poker
with a barge pole).
Sites connected to UK betting companies like Coral, William Hill
, Ladbrokes and Paddy Power Poker are also pretty secure and reliable.