Mk1 and Mk2 Gti
URQ
Audi RS2 ....blub blub blub
Integrale
Honda Type R
AE86 Toyota
and Sierra Cosworth
I think the best investment is to pick the car that people will seek to purchase on retirement, saying "I always wanted one of those back in the day", or "I wish Id never sold it" mentality. Look at GTHOs in Aussie right now. Booming global economy helped as well
Seriously though, I think modern cars (post 60s) just don't have the amazing custom coachwork of the 20, 30s and 50s to make the amazing values that some of those cars do today. Names like Zagato, Giha, Bertone, Allemano, Sallot and the like. Pininfarina the only true survivor that has its designs regularly reach production today.
I looked at importing a 1960s Zagato bodied Fiat 750 (double bubble) from Aussie in the mid 90s, that was listed for 7k with blown engine a but just didnt have the cash. They sell for 10x that now.