In the past I would have immedietly said the Ace Combat series, I started playing with 4, but with each game they got more and more and more like JRPGs, in Ace Combat 6 I was seriously embarrassed to be caught watching the cutscences (and I didn't want to), I ended up muting the TV when someone walked by. It really ruined the whole point of the game, which was supposed to be the gameplay. I still say that Ace Combat 4 was the best.
I also enjoyed flying in Battlefield series games. And the only reason I don't say Silent Hunter 4 as the best is because most people wouldn't consider it a flying game. Not that I would either...