I posted my first car earlier in this thread just because of its uniqueness, but probably my "funnest" car was my '79 Corvette. Bought it for $5K and immediately dropped a $5K engine in it. And of course I had to have the huge sidepipes just so people knew it had a $5K engine in it.
Another interesting point - I bought it in Oregon in 1997, so no sales tax. Then it was my daily driver for 6 years (okay, not daily... I drove the Squareback to work about a 1/3 of the time). It was going to need new hub assemblies in the rear (about $1000 per side if done right) and it had always needed new paint, so I finally sold it about 2 years ago. Sometimes I miss it - but then I remember it had to run 93 octane and got 12 mpg on a good day...