This is actually really tough. One of the reasons I love the Turbo above all other consoles is it's fantastic use of color and the background art. This list could be pages long and still not complete. So this is what springs to mind right off the bat:
Psychosis/Paranoia - Stage 3 ("Lego" level), 4 ("Tomb" level), and 5 (final level)
Gate of Thunder - Stage 5 (Crown)
Castlevania/Dracula X - All
Beyond Shadowgate - All
Terraforming - All, but Stage 3 (or was it 4?) with the "Mode 7" effects tops them all.
Rayxanber II - Stage 3 (giant mechanical spider level)
Psychic Storm - Many of the levels. Been a while since I played through this so I don't remember specifics.
Star Parodia/Fantasy Star Soldier - The ice level and underwater level are best.
Final Soldier - Stage 7
Space Harrier - All. My favorites are the "tunnel" levels as well as 3, 7, and 10.
Fantasy Zone - Many of the levels. Mainly the ones that extensively use very pastel-like colors. Stage 2 is beautiful. There's another stage later in the game that takes place exclusively in the clouds that looks great, too.
Salamander - Stage 3 (fire tunnel level)
Parodius - Pretty much the whole game, though some portions are better than others.
I could go on and on and on.