For a low budget approach (under 25k USD) I would say a 1975 Dino GT4. They are fairly fast and handle well.
Looking at the best bang for the buck(performance wise) in today's market I would go for the post 1988.5 Testarossa. Still pretty fast by today's standards and can be had in reasonably good shape for 60 to 70k USD.
SPASSO 308 V-12 Conversion click here. "Oh yes, that is one of the very best ways to break these and it is also pretty dangerous. You really shouldn't do that". - mk e.
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.
I think the 308 series is the best which is why you are now seeing the value of these cars increase (at least from what I've seen over the last 6 months). The F355 could become this bang for buck Ferrari in the near future as prices continue to fall (which they aren't but will I imagine).
About a year ago, I came close to buying a 365 GTC/4. Great sound and performance. The one thing that was missing, which probably sounds silly, is the metal gate on the stick. To me, it just isn't the same driving experience unless you are running the gears through the metal gate.