Interesting question, especially for the answers: I too would like to know which is the simulator with the best AI, since I play only single player.
It seems to me that they are all mediocre: they do the same evasive maneuvers, the same incredible things and the same stupid things. There is some minor difference here and there, like Rise of Flight's "William Tell" tail gunners hitting you as their plane crashes now wingless, while those from Battle of Stalingrad look much more human. Here, this is the point: I think we really like AI if it behaves like a human being: this study was the subject of cybernetics, today it seems that this word doesn't even exist anymore.
For the record, I currently have IL-2 1946, IL-2 Cliffs of Dover, Battle of Stalingrad (the series, including Flying Circus 1), Rise of Flight United and Falcon 4.0 BMS installed (the latter they say has a great AI, but I was overwhelmed with learning difficulties and couldn't verify it).