Smash's selection of Sonic music is pretty underwhelming in my opinion as on top of every game featured only getting 1 or 2 songs, out of the 21 music tracks that have appeared in Smash, only 4 of them aren't either the game's main or level 1 theme, and the only spin-off represented is Sonic R (debatably also the Storybook games).

This is effectively the list of songs I'd include, assuming there were zero limitations/issues on what could be added.

Only rules is that I limited it to a maximum of 50 songs (as that is currently the maximum track list for a third-party series), and it was done on the basis that every song included in previous Smash games returned.
New songs are denoted /.