For me it would be BA, their Club Class product is excellent and they're competitive on price. I have a business trip in late September (it being more than 5 hours allows me to purchase a business class seat) and a BA business class seat cost my organisation €2300 taking FRA-LHT-JFK. Here in continental europe you do seem to be able to make a substantial saving if you're willing to make a stop on a transatlantic journey. If I'd taken Lufthansa direct from FRA it would have been €3100. If I'd started my journey at LHT and flown direct to JFK with BA it would have been about 2400 UK pounds.
Virgin isn't really an option for me as they are a point to point airline though if you're beginning your journey in London I would expect they are very competitive on price.
I wouldn't even consider a US airline for a business class ticket.