For trans-Atlantic I’d rank them AF>KL~DL. For me AF seats are by far the best (don’t think the fly the 2-2-2 layout to US / CAN but may be wrong). DL seats are pretty awful, including the suites (it’s a 2-3 foot high door that anyone walking by can easily see over and watch you laying in your coffin, I consider it “privacy”, but if that’s what is important for you then DL is going to be the best.) KL 777 is bad but the 787 is rather nice. However have had several KL flights the past few months and the food is mostly inedible, DL is occasionally better and AF is usually decent if unmemorable. I have incredibly low expectations of airplane food, but am still often disappointed by all three carriers (and most others to be fair). Service I’d probably give a slight edge to KL for consistently good, with DL having the widest range from great to horrible.

It really depends what you consider most important, but they’re all mediocre enough I would choose whichever is cheapest (or KL if you want to collect houses). Or pick one you haven’t flown yet to give it a shot and make your own opinion, I’m sure some people love the food on KL and hate the seats on AF.