I'n gonna tell you right now: nothing is ever gonna top Speed Highway Act 2 (Modern).
This is easily the best level in the entire Sonic campaign. The sense of speed is truly strong here, and it the best layout of any level from Sonic Generations. But, the part that really makes this level stand out from the rest of the levels in the game is the highway segment where you have to "drift" Sonic down the highway. Not only is it a great mechanic for the level in general, but it also literally sounds like car from an anime drifting when you drift Sonic. It's Initial D but in Sonic form.
This level is just incredible. Easily the best level of the whole Sonic campaign, bar none.