Quite a wide open range for opinions to fall into.
First off, are you looking to build a car to resell at a profit? Are you looking at something to retore to participate in teh hobby of restoring, or are you looking to have something that you can enjoy daily? Do you want to show it, or drive it? Do you want stock or custom?
Ok, on to opinions. Personally, I'd say mid '60s Lincolns and Caddys. Always classy, relatively easy to work on, even though they are complex. Cheap enough to buy into for a decint car that needs work. can be left stock or customized, and eith er driven as a low cost "fun" car or spent stupid money on for either a 100 pt restoration or show winning custom.
you could also go for some offbeat full size cars like late '60s Impalas and Caprices, or cars like that '65 galaxie that was posted in the other thread.