I have a relatively minor problem with the way the question is posed, strictly on the body style/generation changes....in that the '82-4 share the same drive train, so it blurrs the line a lot....over a 3 year time frame....
but changes are not even across the line as some changes affected suspensions more radically than say engines, and of course the ever increasing use of electronics beyond simple engine management I personally consider to be a maintanance/electrical nitemare in the making...I have allready witnessed that on early C4 cars....also the increasing use of cheap plastic interior parts make 'restoration' and/or just 'holding it together nicely' allmost impossible...things like console box covers, door panels, dash trim....
seats that do little but fail a lot...airbags are nice for safety...but when need replaced...well....;-(((.....and that can be more than the rest of the repair..depending....

so when asked which generational change is MOST important....
I would reply the 81-85 changeover....for the DPFI and O/drive trannies...
it changed the entire concept of engines, fuel management, and cruising capabilitites.....of course the wheels/tires are in there too....
not that any one change occured each year...but in those years was easily the biggest changeover....some positive in the drivetrain/chassis areas....but also more negatives than should have been allowed in the body/interior/weatherstripping area....

now for all the vaunted chassis/frame stiffness and other improvements in the C5 I find them mostly guilt on the lilly, by NO means do I mean the car is a POS or something silly, but the biggest single change is additional electronics with attendant problems...and aluminum engine block...wonderful...same technique as a '63 Olds 215 ci engine....what's really 'NEW??"....the rear suspension and water formed frame are interesting, but I wonder just exactly is the engineering numbers...hard numbers on the improvements.......

I feel the question really revolves around the electronics/computer breakthroughs...and that is all early 80's crap...nothing really new there....

GENE

 

Gee I would have said 67 - 68: the inroduction of the optical light monitoring system and the vacuum assisted wiper doors. :banghead:

Actually it's a toss-up between 62 - 63 (introduction of IRS as the main difference and creating a world class sportscar) and 96 - 97, a completely new car with nothing carried over from the previous model.