Final Zone

From Sonic - Wiki on Neoseeker

Final Zone is the final level in Sonic 1. There is only one act to this stage, which is a boss battle. There are also no rings to collect.

[edit] Boss Battle

Dr. Robotnik will appear in one of four pistons. On each round, two of the four pistons will shoot out, one having Robotnik inside. The one with Robotnik inside is the one you need to hit, while avoiding being crushed by the other pistons. Following this, some generators at the top of the screen will shoot out balls of electricity. After dodging the electricity, Robotnik will attempt to crush Sonic with the pistons once more.

The stage ends if you die (as rings are not carried over from the last act), or if you land eight hits on Robotnik. A nice bonus is when Robotnik escapes in his Egg Mobile, Sonic can leap up and hit Robotnik, causing Robotnik and the Egg Mobile to explode. The ending sequence happens regardless if you blow up the Egg Mobile.