Now that all the games are basically on the same level, I still like WOI the best, I think the stock campaigns are awesome and really represent the wars well. Next, the original Strike Fighters Project 1 campaign has always intrigued me, since it is fictional and during the Cold War (A part of history I am extremely interested in) the fact that all kinds of things can be explored interest me.
I really don't like WOV, I like the era and the history of the conflict, I like flying scripted single missions, but flying a campaign where I fly up to bomb 1 fuel tank on an air base seven out of ten times kills it for me, even though this is probably how the pilots felt at the time. Although SEAD in a Thud is pretty fun.
Wings Over Europe is what I put all my mods into, for the most part, but i rarely if ever play the campaigns. I am just so burned out on NATO vs.Warsaw Pact in Germany. Give me NATO vs Warsaw Pact in Norway, Turkey, Middle East, Antartica, anywhere I'm down, just don't have any interest in Central Europe at this point.
However, I have to say my favorite Thirdwire "title" is the all inclusive Operation Tainted Cigar mod, that I have updated myself here and there adding new things that have been released since the mod. I have probably flown that campaign from all flyable squadrons. Most of my campaign flight hours are over Cuba.
Edit: I also want to add I fly First Eagles a lot, I dig the simplicity and challenge of Great War air combat.