American has been the easiest, especially with all the partners. I'm getting very frustrated with trying to claim any awards on Delta. I still use my Delta Amex, I like all the double miles promotions, but I just can't go anywhere on Delta! I checked in October for flights to Europe for next August. The Delta agent's reply was "Are you kidding? With the Olympics going on?" Kind of rude, she wouldn't even check, and I wasn't trying to go to Greece.
I've checked Continental several different times for flights, and there was no availability. American agents can usually find something, they seem to be more willing to try different ways to get somewhere if the most obvious connection is not available.