The 50's, 60's and early 70's have a lot of collectible vehicles. There's too many to even list right now, so I won't try. Personally, I don't see very many compelling vehicles from the middle to late 70's until the early 90's. There are some (like the old Z or the Mustang), but not many. Commonly, cars from the 80's just plan suck in styling and performance. The boxy cars of the early 80's are ugly for being boxes on wheels. The aerodynamic cars of the late 80's are ugly for being wings-with-bubbles on wheels. I don't think there are many collectors trying to score a 1989 Ford Taurus.
I was at the recent car expo in San Francisco. I found there are many compelling vehicles. I think the middle 1990's to now (and hopefully for years to come) is going to be an era with many collectible cars. Vehicles popular in the Tuner market almost already have a collectible status, like the previous Honda Civic. In addition, there are loads of new, interesting models or resculpts, like the Dodge Viper, any BMW, certain Subarus, the previous and new Mustang, etc which will likely stand the test of time.
It has inspired me to hold on to my car and keep it in great all-original condition for the next 20 or so years.