In the real world, it's a hard decision to choose between the F6F and the P-38 as my favorite plane.  I love the history behind both of the planes and their pilots (they are also some of the most beautiful aircraft).  In game, I really don't have a "favorite."  Some tours I spend more time in particular fighters, but I try to spend an equal amount of time in others.  Like I've told many players, you will almost never see me in the same aircraft 2 sorties in a row.
Also, if I would really have to choose out of all the aircraft, the B-17 would be my favorite both in game and in the real world.  The B-17 played a large part role in WW2, but the main reason is my hometown was a training base for B17 pilots.  I wish I could find some pictures of "back in the day"