The first game is abysmal. But since it's actually called Air Combat and isn't part of the shared continuity of the rest of the series, it's easy to ignore.
Ace Combat 2 is very good, just not as good as the later versions.
Ace Combat 3 should be awesome, but it was hideously butchered in the localisation process. The gameplay mechanics are still as good as they were in AC2, but a lot of great features like branching mission paths, AI wingmen and a decent story were cut out.
But yeah, AC5 is by far the best game in the series. AC6 improved some elements (the HD graphics make a BIG difference), but messed up a few others. Especially the story. Ye gods, how do you go from the Razgriz poem to repeating "Go dance with the angels!" every thirty seconds? Still a superb game, but can't touch the sheer atmosphere of 5.
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"I'm Commander Shepard, and you're standing in the biggest library in Citadel Space. Look me up."