I buy and sell vintage cars for a living and here's my .02 on what will always be collectible.
Any car that was popular when it was new, especially if said car became very inexpensive as "just a used car" and "all the good ones" got used up.
Before I start a big controversy, I am not saying that these are the only cars that will appreciate and become collectible.
That said, I think these cars will always have a following:
Any manual transmission Miata
Honda CRX
84-92 VW GTI (can you actually find a really nice one?)
1997-01 Honda Prelude (can you find one that's not lowered to the bump stops?)
Porsche 944 (too cheap for too long - good ones are scarce)
E36 BMW M3
Any Toyota Supra
What do you guys think?