Chemical Plant is definitely memorable, yes, and many would call it iconic. This is an entirely subjective debate, however, and although that is a great song (and fits the Zone it appears in), here are just a few which I consider to be better off the top of my head:
"Ice Cap Zone Act 1/Act 2" from Sonic 3 which have already been mentioned are both absolutely fantastic:
"Mystic Cave Zone - 2P VS version", from Sonic 2:
"Panic Puppet Zone - Act 1" from Sonic 3D Blast - people who have never played this game but have played Sonic Adventure on Dreamcast/GCN will be very surprised when they hear it:
"Hyper-Hyper" - from the Sonic Crackers 1994 Prototype (a later version of which appears in the released Sega 32x game Knuckles Chaotix):
"Electoria" - also from Sonic Crackers:
And my personal favorite overall: "Gene Gadget Zone - Act 2" from the Genesis/Mega Drive version of Sonic 3D Blast. Really shows off what the YM2612 sound chip can do:
"Who Here Hates Those Damn Eagles?" 5/13/01 - 1/8/05. Ninja Gaiden (NES). By Mikage. The greatest topic ever on GameFAQs.