any time period before the hegemony of the plastic bumper… even then a VAN had style
The '80, '90 and '2000 (before Downsizing).
2002-present purely because of the technology.
1980-2000 because all the famous jdm cars 
( silvia , ae86 , skylines…)
and 2010 - 2020 because i love to design modern cars.
60s for the muscle cars, 10s for the supercars
I have to say the starting years: get going in a little shed, welding together spaceframe chassis, building premium cars on a budget - that should be pretty exciting!
The early years are often the most exciting for me. Imagine to able to start back around 1890/1900. Would be freakin’ awesome with the wizard type of engine creation!
I dont know if you really mean IMAGINE, but the game will start around 40s-50s, so 1890s 1900s wont be possible, unless modded in by a modding group.
That would be a huge undertaking, but I would say that it would be worth it. A mod for the future would also be amazing.
Hence the imagining part since it doesn’t exist, at least not as of yet 