I'm going to chip in here - I'm writing a book on the 916 by the way.
I'm sure the cars are on the cusp of classic car status. Their successors will soon be discontinued, so they are probably old enough now, and look different enough to make a credible classic car choice, rather than simply being a used car. Their predecessors are starting to appreciate and if you are in the market for a usable sporty coupe or spider from Alfa Romeo, these are now looking like a really sensible choice.
The Phase 1's look a little bit more "classic" now - the matt black lower body is quite old fashioned, and a Phase 1 12V spider in metallic red with a tan leather interior would be my choice. But I reckon the 3.2s will be the most valuable in the long term. They sold in very small numbers in comparison to the Phase 1 and 2 and are nicely equipped. The 3.2 litre engine is fantastic as well, and for me just shades the 3.0 (though my experience of the 3.2 stems from my old 147GTA rather than a GTV or Spider).
If it were my choice I would buy a 3.2GTV if I wanted a car that might appreciate, though I do prefer the Phase 2 3.0 GTVs without aerokit design-wise.
Having said all that I think they will all be classics in end!