Keep in mind that early 240 and 260Z's hit rock bottom in the early to mid 80's. Not desirable at all at the time...today an excellent show car example can command up to 20K or more. So what might be considered a un-desirable car today might be an unseen classic in a few years.
You are seeing the same returns on 70's model Vettes right now too. Good examples are getting good money.

Considering this, and clean stock CRX Si will most likely be highly sought after when we are all older and have money and want to relive our roots.

But my chosices would be '93 Cobra Mustang, or any clean 5.0 Mustang, especially LX 5.0's convertibles (anniversary edition)
Of course the 300Z TT is already a classic. I'd even say that the first gen 240SX SE's will be sought after, convertible versions being most rare.
TT Supra, Impala SS, '93+ Miatas, Camaro Z28's, Integra GSR and Type R,
of course Viper GTS, ZR1 and Grand Sport Vette. Thats all I can think of right now.