would be my recommendation. It is invariably KLM and following a very unpleasant experience a few years ago on an ancient 'combi' B747. Surely crew, terribly uncomfortable seat, having to have main-deck cargo offloaded before they would open the doors, etc I avoid them like the plague.
I also avoid BA, despite them having the best seats. Their OTP is pretty bad (in my experience), T5 is a pain and connecting to any other Terminal is even worse, and their treatment of premium passengers is bad. They really don't give a
about their best paying customers. Their service is inconsistent at the best of times, which is not now. (Avoid LHR-IAD flights unless you are a masochist.)
You don't say where in Europe, but I would pick the European national carrier who likely will fly non-stop; followed by CO, if they also fly non-stop; followed by any European Star carrier's connecting service; followed by AF's connecting service.