Probably all US airlines. They always have to be called twice, always have to have the message repeated and always use ambiguous non standard RT (well maybe not always but enough for it to be a safety issue).
I have never flown in the USA but I guess the US controllers must be brilliant to stop US pilots from bumping into each other, especially considering how busy the airspace is there.
[ 02 September 2001: Message edited by: autobrakemedium ]