You could also do UA to LGA via IAD, which would at least give you a saner arrival in New York.

There's also TP via LIS, US via PHL (or even getting the train from PHL to Penn Station) or AC via, well, Canada.

You could consider splashing out for F on the outbound only, allowing you to enjoy the much-lauded LH First Terminal in FRA.

Otherwise, I would rate the European airlines as LX > LH (new C) > SK (new C) > TP > LH (old C) > SK (old C).

Another quirky alternative is the Privatair service from DUS-EWR - you can connect to DUS from LHR and LCY. It's an all-C service, and has great availability - some love it, some hate it. I personally really enjoy such services, had a great flight to TAS in an A319LR.

Note that if you're connecting in FRA, DUS, MUC, CPH, ARN, ZRH or GVA, you can start your journey (if you're in London) from LCY, which can be much more civilised that LHR, if the location suits.