BA economy v KLM and Lufthansa
If it has to be Y, I would let price and schedule decide which one I would go for. All three airlines mentioned by OP are a decent product. The economy comfort on KLM as mentioned above makes the KLM experience a lot better. Increased legroom, better recline, slightly wider seat. Cost per leg without status is somewhere around €100, so you want to factor that into your overall price. Please note that this is in no way comparable to WT+, food and drinks etc are all standard economy fare.