Everybody seems to be avoiding the biggest question

Which airline has availability.

All of these are useless if you can not use your miles, or special system wide coupons.

From my little experience I tend to think that of the US airlines flying to Europe, probably:

UA - has best availability so you might want to go with them. They also have many flights to Asia if that is also where you would like to go internationally

AA - probably number two for avaialbility. Rather weak in Asia though

NW / DL / CO - definitely at the bottom from the seats I was searching. However recently I found a slew of flights DTW-CDG that had upgradable space a plenty.

Maybe others can chime in here, but AVAILABILITY is pretty limited on some carriers especially if you are looking at bringing a business partner, friend or spouce along with you. A couple of friends - a real challenge....