If we are using the term 'worst' and linking it to the number of complaints I have made, amount of late running/early running, reductions to services, price increases that are at times above proportion, drivers who have had stand up arguments, customer services who provide innacurate data and fail to adequately respond in addition to experiencing run-ins with drunken or stoned passengers - then GNE are the worst.
However, my usage is disproportiately larger than other operators.

Based on limited travel (the last being 8 years ago), Coastliner is my favourite by a country mile.
However, if I was to use them as frequently as GNE, my answer may be totally different.