AA seems (IMO) the airline we've had the easiest luck booking f.f. seats with. They also allow stopovers in both the USA as well as in Europe and an open jaw.
I think Delta is difficult to work with unless you have elite status like Marc and Peter have. We rarely find any open seats with Delta no matter how far ahead we try. I think the Delta phone line we use must flash "PEON" when it rings!
