SA1 was a good game for its time. I can't stress the "for its time" element enough. Time hasn't been all that kind to it, but certainly at its release in 1999 it was incredible. It told an interesting story. It made good use of its 3D environment. It was immersive. By today's standards, of course, it's archaic. The camera was more of a threat than any enemies and the inclusion of certain superfluous purple cats was mind-bending.
Then there was SA2, some abhorrent nightmare that inexplicably received laudation. The story was cripplingly bad, foisting a mood of sci-fi/techno grit on a series that had never been about any such thing. Superficially, it was about saving the world, as Sonic has always been about, but SA2 went over the top, presumably just to be edgy and appeal to the angsty teen crowd of which Shadow was emblematic. The execution of Maria? The spiraling descent into madness of Gerald? Robotnik's utterly bizarre decision to blow up half the moon? Yeah, that wouldn't have caused calamity to the world. If anyone can also explain why the ultimate life form is a black hedgehog, I'd love to hear it, especially when the ultimate life form "prototype" is a tremendous brontosaurus with giant purple pimples.
So much of SA2 was committing the same errors from SA1 without trying to fix them. The camera was still unkind. The level design became significantly worse, relying increasingly on bottomless pits in lieu of actual challenge and following absurdly linear paths that did little to encourage exploration. SA2 refused still to answer why Sonic et al. were running around in a human world. Just for fun, SA2 also made the treasure hunting stages worse by limiting the radar's capabilities, as if they were any fun to begin with, and made those stages unnecessarily larger for a bit of controller-biting rage inducement. That's independent from asking why the Master Emerald needed to be broken again. Grinding was a complete and utter sell out to Soap shoes, and between that and the declarations of Awesome! or Great! or Wicked! or whatever other stupid phraseology appeared for destroying many enemies in succession, it felt like this was Tony Hawk: Sonic Edition. That only happened when you were playing as a hedgehog, and considering the name of the game, one would think more time would be spent playing as Sonic. Alas, no! Time had to be given over to putting Tails in a mech for stages that did not contribute anything to the plot.
And that music. That horrid cacophony. There are things that should not be and the SA2 soundtrack is one of them. A majority of it is Jun Senoue thoughtlessly yanking on guitar strings and the rare piece of quality music is lost amidst a sea of discordant notes.
The title of SA3 was originally intended for Sonic Heroes. Heroes is an alpha build of what SA3 was going to be, which is a horrifying thought. The name SA3 was also quietly floated in Japan as the name for Sonic Unleashed before it was changed to Sonic World Adventure. It won't happen anytime soon, and probably never, because the name "Sonic Adventure" is remembered through deeply nostalgic lenses, for better (as with SA1) or worse (as with SA2).