Sonic the Hedgehog 2
After the huge success of the original Sonic the Hedgehog in helping the Mega Drive/Genesis catch-up the Super Nintendo. In fact, in some markets the Mega Drive outsold the Super Nintendo, thanks partially to the success of their brand new mascot. So it was no surprise when a sequel was released sixteen months later in November 1992.
The biggest change to the series is right there on the cover; the introduction of Miles Prower, more commonly known as 'Tails', as an ally for Sonic. He will follow you everywhere and not do much, but if you have a controller in the second port then they can control Tails whenever they want. A 2-player mode was introduced, but it's largely forgotten about. There are more Zones than the original, but now they are just two Acts. There are other changes, but I'll get to them in due course.
I'm going to supply a video of the first two levels (the first because it is the first level, and the second because it is the Chemical Plant, probably the most iconic level in Sonic 2).
My experience with this game doesn't go far beyond Chemical Plant, though I've seen the game through many times. This won't be as easy for me as the original, but a bit of challenge never hurt anyone, eh?