Given the fact that the name "Genocide City" is due to an misunderstanding, early fan interpretations of the level are completely different from the original concept.
Sonic 2 Long Version's interpretation of the level. The graphics were taken from one of the levels in an SNES game, and the enemies are from Chemical Plant Zone.
A hoax screenshot about Genocide City, which used an unused sprite from Simon Wai Prototype that was later be confirmed to be merely a leftover from another game. (Image from Sonic Retro)
My personal favourite is the Genocide City Zone from older versions of Sonic 2 Delta (the latest version had remade the graphics to match closer to original concept). It's just Chemical Plant Zone with recolored Oil Ocean Zone backgrounds but I love the feel of it. Oh, and the music is cool.