AA from JFK will give you the most sleep (if that is important). New AA seats are better than LA seats. LA seats are fine enough, but not aisle access for all. LA wine is better than AA. LA crew are usually but not always better than AA. Haven't flown UA in years. I'd take the best schedule. From ORD, you're probably best off with AA or UA so you have the longest comfortable flight. MIA (LA) doesn't provide too long of a flight, MIA connections aren't typically delightful, and JFK (LA) is going to require a change of planes @ SCL.