If you're looking beyong the initial purchase price - which seems the sensible way to answer, then you can't seriously look at the Testarossa the 400/412 or any other 12 - they're just too potentially expensive to maintain, and heaven forbid something goes wrong with them
I do agree, however, with the 308 GT4 - a fantastic car which can be had for very little money. That would be top of my list for value - but don't think that that's just because you can buy them so cheaply - they're a fabulous car too.
After that, the 348 is an awful lot of car for the money. They're really at that point in their history at which they're not going to depreciate much more before they attain classic status, and the good ones start to increase in price.
For older models (in which you just have to accept the potential financial nightmare of a V12 engine) - the 330 GT Series II can be had for very little money. Also the 365 GTC/4 is a serious car, and an absolute bargain - probably due to it being forever overshadowed by the Daytona which was being sold at the same time.