I don't have a crystal ball but I don't think 944/951/968 are a good "investment", I just don't see them going up in price.
Ask me how I know.
Now a 356 or early 911 maybe, however I don't know when 356 prices are going to top out.
Hate to say it but the Karman Ghia probably is the best bet.
I think the 951 is the most fun for the money Porsche (I'm a little biased) but not a great investment.
I suppose the 88 silver rose or 89 951 in excellent condition
may be considered an investment.