i'll stick with 91-91 1st gen
94-96 2nd gen
97-99 3rd gen
here's my reason why. not 100% sure on all of this, though.
1st gens had pop up lights, 5-speed manuals, more testerossa-like vents in the front bumper and side skirts (3k's), steering wheel mounted radio controls, active air dams, etc. the ES trim was only made 91-93. the base stealth (91-96) was an sohc engine. both ES and base had trim on the sides of the car. r/t // tt had side vents, and vented b-pillars.
2nd gen. they did away with some features from the first gens. active air dams, i believe, kept the active aero spoiler. projector lights, instead of popups (94-98 were the same). HOWEVER, the light pods vary somewhat between all trim models and between generations.
http://www.3sxperformance.com/faq/99/index.asp keep this in mind if/when you want to make changes to this area. manuals are now 6-speed, side vents are gone on the stealths, 96's have an improved boomerang spoiler. also, only 95/96 are the only years the spyder is made. overall look of both 3k and stealth is smoother, and has less trim.
3rd gen. stealth is no longer made. base 3kgt is now an sohc, not dohc. new front bumper, hoop spoilers, got rid of active aero, no more all-wheel steering, charcoal-grey interiors are gone. minor changes for 99 models, front bumper, no climate control, modified 2nd gen stealth lights, vr4 combat wing, sail panels, rear garnish.
some may argue that the 99's should be considered a seperate generation, and that 94-98 was 2nd gen, but i disagree. an 86 corvette or a 96 corvette, they're both c4's, even though they look subtly cosmetically different. after 96, the stealth was no more, and the base 3k became sohc. THAT'S a change.
-thadd.