"The Best: Zero and 5
I can't decide which is the very best. I need to say BOTH are. Great storytelling for each, lots of fun missions for each, moving moments especially in 5, vast variety of playable aircrafts especially in Zero, with satisfying gameplay. The undeniable masterpieces, 10/10 games for sure.

2nd: 7
I haven't touched VR since I don't own PS4 but the base game itself is already awesome for me. The missions are challenging, the graphics and sounds are breathtaking, even MP is not that bad once we get used to. The story is kinda unpolished or too rushed because of development delay but still enjoyable as a big service for PS2 games' fans. I have totally no regret for pre-ordering this awesomeness.

3rd: 04
If this were the first game, I might put 04 in S too. Simplicity is this game's specialty, the storytelling is golden. Yet, they put one great invention to the series: selective SP weapons. That makes how fun Ace Combat game is now. Significantly great game indeed, also it features the most powerful, terrifying protagonist: Mobius 1.

4th: X
Completed X way after the release, and now I still shame on myself for having missed this great handheld experience. The 04-like story but with almost the same flying mechanism of 5, which turn out to be the match in heaven. Surprisingly challenging and full of charm for a PSP game. One catch is that the voice acting is kinda goofy. I'm talking to you, the NAIAD crew.

5th: 2
Zero is my first game in the series and I have no nostalgia for the games in PS1 era. HOWEVER, I can recommend playing 2 for everyone. The gameplay aged so well that it is still enjoyable. And it features probably the best rock soundtrack Namco can offer. Surely deserves the remake, and they did for 3DS... which I haven't played.

6th: 6
Like X, 6 has its own charm. The vast combat with tons of fighters is surely astonishing. The ally attack system successfully makes me feel "Man, I'm leading the entire force!". But Xbox exclusivity killed the population. The poor storytelling did too. Not the worst game ever, but failed to be a great one.

7th: AH
No matter if the entire Ace fanbase still see AH as the huge disgrace, I defend it. It's far from the worst game. It's a freaking spin-off, for trying new things. I don't hate DFM. I don't hate the Chopper and Bomber missions. The legacy AH achieved has succeeded to 7 as a better cutscene quality, beautiful graphics and PSM mechanism. I can't deny the story is boring though. They definitely couldn't decide their mind, making it super-realistic or over-the-top anime-like. But they tried. We should not deny that point.

8th: 3(JPN)
To begin with, I'm not a big fan of super-futuristic sci-fi. And for me Ace Combat is for having fun with irl crafts and some overpowered Strangereal hyper-jets. Surely I like its atmosphere but 3 just can't satisfy me. And the gameplay is not good as 2 imo, especially the thumb stick issues that the game actually can't recognize analog input. Not a bad game at all, but I can't see why heck a ton of people are so eager for the remake.

9th: 1/Air Combat
AC1 just aged badly unlike 2, that can't be helped since it's one of the earliest titles for PS1 though. Great tech demo? Yes. Great game? Not anymore. This game might require HOTAS more than 7, since playing it with d-pad is sheer awful.

The Worst: X2
After the great debut on PSP with X, how come could they make this game this bad? The graphics looks inferior, the gameplay is painfully slow, the soundtrack is OK while they don't know the proper timing for the theme song, and worst of all the enemy AI for ace team is... just cheating. Imo Sulejmani is WORSE than Akula. Taking down Akula is not so hard while he... I just go "F*ck you Sulejmani"."