IMO too many people focuses on secondary aspects like story, character design, etc. For me the fundamental components of the true brilliant Sonic experience were three: momentum-based gameplay + great level design + great music/art.
And no game since the Genesis ones have had all of that. No game since the Genesis ones feels fun only by moving the character and goofing around (just like the Mario games have felt from its day until now).
I felt like the Sonic Advance series of GBA games were pretty damn good, not my favorites but some of the better games when compared to shadow or heroes. Fun momentum-based stuff and pretty much the entire cast of characters minus dialogue and silly story stuff.