Ace Combat 6 seems to have got the lowest rating out of the series. I think Ace Combat 4 was the second real video game I got (on a game console, the PS2), after Gran Turismo 3. Of course, I do not find the game dull or boring since I have so many of them and so many hours on each. But, at first, I remember I did not like the game. I gave Ace Combat a second chance and came to like it. My main interests are aviation and space (flying mechanical objects?) - and I suppose Ace Combat goes with the flying interest. Ace Combat 5 probably has the best story, and Ace Combat 4 has the best gameplay in my opinion.
Although, I do not have any modern combat flight simulation. I have Combat Flight Simulator 3, which I like but gets boring after a while. Same thing applies with Flight Simulator 9 - but FS9 is probably my favorite just to fly on, I would get bored of just flying after a while on any game. I suppose I prefer more arcade-type combat simulators.
I also did mention Gran Turismo, my favorite game (GT4) and my favorite series. I do prefer driving simulators rather than driving arcade games by far. I wonder if anybody else here plays Gran Turismo.