Robotnik just went badass
This would be Robotnik's theme if Sega gave him the proper attention. It sounds dark, epic, and HUGE!
Something about this sounds dirty. Like Industrially dirty. Like wherever the fighting is happening is covered in grime and soot.
All the electrical sound effects and zooming instrument sound like an attack being launched. It's great for the whole final fight sound this song is known for. That said, this really does sound like the toughest fight of Sonic's life if they'd continued to write in the Genesis style, which seemed somehow darker.
I don't know really what else I can say other than how epic and awesome this is. but I especially loved the pipe organs and how they just sounded sinister. You know you're fighting a villain when this music starts up!
I get the impression this fight is happening in an older portion of one Eggman's facilities, likely in the utility sector, and the fight is happening among pipes and steam jets. At the 2:00 minute mark I see Robotnik escaping and Sonic goes super to catch up with him. At around 4:30, Sonic delivers the final strike, knocking Robotnik out of the sky and saving the world.
...
What?