I think a new Mania would be a bit too soon, and I don't think it'd be too different of a game to outweight the other options. Plus, objectively, you're getting far less out of it compared to the others in general size of the game/number of games.

A cartoon, while it'd be fun, wouldn't do much to help the franchise. And it's not an area of Sonic that's really hurting. As the shorts were always fantastic and Boom was pretty good for what it was.

I'd say Adventure remakes would be the best choice overall. You're still getting good 3D Sonic games, while also perfecting a classic in the series. New players can see what made people love early 3D Sonic while making the games much more replayable, as well as bringing back playable characters other than Sonic. And it would probably have the best chance at revitalizing the series for future games.

Plus, Adventure remakes should carry over to an 06 remake as well. (Considering the game is pretty much SA3 anyway.) And I want to see that glorious mess brought back as something unironically good.

They're all undeniably a Holy Grail considering the franchise's history, but if Adventure remakes also includes remakes of Heroes and Shadow The Hedgehog then I'm going to pick that because did I mention how much I like Heroes' art direction and gameplay and how much I enjoy running around as a Edgy Rotary Cannon-wielding Half-Alien Cartoon Hedgehog with Rocket shoes?