paddy>wh>boyle>celeb - betmost belongs to wh and should be as safe as wh.
celeb being on last place is because they aren't a big bookie like the others, still they are around since 2004 and by no means an unsafe skin to play on - with all problems iPoker does have (and had in the past), players have always been paid and no skin has gone bust ever.
Even players from skins like propaganda poker always got paid