In my previous occupation I worked with a US company and made many transatlantic trips. My recommendation has become:
Prefer direct flights It is faster, no connection to miss (due to immigration queues) and less chance that luggage gets lost.
About the airlines: I found that BA has most consistent service standards on the route; KLM can be more fun, especially when you speak Dutch
. I rate Delta and AA slightly under BA and KLM (must admit I have very limited experience with AA); then United and US ranks last. Beware that the "regionals" that fly the smaller planes in the US can be "glad to be off" bad.
My recommendations for airports to connect in: Detroit and Philadelphia. Chicago is not too bad, but has very limited facilities behind security in the International terminal.