You have to look at what our generation drives, or would want to drive.
Currently muscle cars are expensive because people in their 50's who always liked these cars in the 1970's can now afford one.
Based on that:
BMW 1M
BMW E46 M3 ZCP (in good condition)
E92 M3 in some type of nicecustom color (BMW individual)
Subaru STi, not the standard ones but the STi Limited, STi Spec C, etc.
Mitsubishi EVO 9
Mitsubishi EVo 6 (if you can find one)
Mustang BOSS 302
Camaro ZL1
C5 Corvette Z06
Audi RS4
Audi S4
Mercedes C63
Lotus Exige
Lotus Elise
Porsche GT3 / GT3 RS (not 50K, but...)
that said, I would like any of these cars any day, might be a little bit subjective but I find these cars could hold their value a lot.
My boss had an E30 M3, Canadian car, never driven in rain, snow, or even sunshine, always under a cover. It had 9000km when he sold it a few years ago. I think it sold for around 50 000$. Now THAT would have been something to buy !