[EvanZR1]

1990:
One year only body style, one year only wheels (salad shooter). Super narrow sport seats with electric recline (36" waist max, a few 91s (like mine) also got these if they were rare/unusual color interior)
1991+:
New front bumper and belt-line trim. Sport seats are same look as 90, but a little bit wider and move to manual recline adjustment. Change to saw blade style wheels.
1992+:
Traction control added. Emblems on both sides of hood (although many earlier year owners add this). Exhaust tip change (of course exhaust is one of the most common things owners change).
1993 specific: 40th Anniversary Edition (appearance only option).
1993+: Increase to 405 horsepower, but car got heavier. PKE Remote. Passenger airbag. Oil & coolant temp added as options in digital dash functions (believe it was 93, may have been 92)
1994+: Another seat change, now the widest seats in a C4. White dash lettering. 5-spoke, A-mold wheels. OBDII port, but OBDI ECM.
1995: Possible to get a "double Dunn" engine (Dunn heads on both sides, more of a unique conversation piece, maybe worth a few (4-5?) HP), shifter lift ring eliminated and change to "crash through"

As Marty said, changes are actually minor, evolutionary, not revolutionary. Also, many of the "improvements" to the C4 were done as running changes and not tied to a specific year. So things like felt pads in certain places to help eliminate rattles were done once they identified an issue and had a solution, not waiting on the next model year to implement. In general, I typically say buy the latest model you can get with all else being equal. But on ZR-1s, that tends to go out the window as it's more about: what's available, color, condition of car, mods, specific things you value, etc.