I avoid any transits at US airports, especially NYC area ones. It's a zoo, and JFK is very prone to delays.

AMS is the best to make a correspondance at (hence KLM) assuming they get their act together this summer.

Hard product, like others said, the PE product is probably better on UA. KLM is hit or miss, not all their planes are retrofitted yet. AF has a mediocre PE product unless it's the newer ones on the A350 and 77W with the new J class (still being retrofitted too). And I'm not sure why anyone flies LH, plus FRA T1 is either confusing or a mess depending on your luck; and we've been having our own share of strikes in Germany lately.