BA, KL and LH are my three most frequented airlines. For a long-haul in Y, my choice would be KL, BA then LH.

As others have posted, the KL Economy Comfort product is a fairly low cost additional cost on top of the Economy fare and you can choose to do this at any time, irrespective of status. If you have status you get a nice discount, or potentially free if you have FB Platinum. If I was not prepared to buy up to Economy Comfort on KLM, then I would probably go with BA on the basis of my status benefits.

For me BA, hold on to 2nd place because I'm always hopeful of an upgrade (ha!) and for whatever reason, I feel I'm already home as soon as I board the aircraft on the way home. I don't get that feeling with KLM.

I'm not the biggest fan of LH, though to be fair, they are quite good and you know exactly what you're going to get. Their pricing is usually very keen - my company travel policy forces me to take the cheapest route within certain parameters. I have had more IRROPS with them than any other airline, including my only ever invol downgrade (from Business to 53B on a 747). They were very proactive in paying me the quite substantial compensation though.

This doesn't affect the OP, but for the benefit of other FT'ers, I would be very wary of a route via FRA or MUC between December and February in case of weather-realted IRROPS - despite popular belief, German airports also grind to a halt when the snow comes.