You know, this is funny. I guess it's a perfect example of "your mileage may vary." Because I've always had great luck with CO. For one thing, their phone agents always have gone out of their way to try to find a way to accommodate me, more than anyone else. I've gotten flights to Europe with less than 2 months notice before, and in May and June. I got a Delta award ticket to Venice with no problem, either (for early June, as I recall). Most recently I got Delta to Athens, although it was January and I had to check a lot of dates (but it was only 6 weeks or so prior).

The bottom of my list would absolutely be US. Their agents are rude and completely unhelpful. You have to prod them to get them to check alternatives.

I also would say that domestic tickets on all of them are much easier than transatlantic. But everybody probably knows that.