Tough call.
I've had a lot of fun cars. The one car out of all of them that was always an absolute blast was my bright yellow 1978 FIAT X1/9 5 speed with the Fuel Injection. Original 13" pizza cutters were replaced with a set of 15x6.5 Enkei mesh wheels wrapped with 195/50/R15 Dunlops, upgraded front and rear sway bars etc.
Not even remotely "fast", but to this day, the most ridiculously nuetral and perfectly balanced car I have ever driven. The grip was outrageous. The car quite literally lacked the power to get out of control. It wasn't slow, but that little engine simply could not trump the handling. I would own another in a heartbeat.
Close runner up: in 1988 while living in Dana Point, I wanted a car I could put surfboards in and take to the beach that I didn't care if it got filled with sand and constantly straifed by incontinent Seagulls.
Drove past a house with a shitty looking 74 Pinto wagon with a for sale sign on it. $400 OBO.
Perfect.
So I go knock on the door, surfer bro opens up and looks like he's halfway through a massive bowl. I ask him about the car. Does it run? Will it pass a smog check? Etc...
"Yeah brah! It runs bitchen! Check this out dude!"
He opens the hood, and there sat, complete with chrome valve covers, a nearly brand spanking new Ford 289. This beast is coupled to a 4 speed manual out of a Mustang. I crap you negative.
He had been trying to sell it for months, and the shitty fuel economy and high gas prices were too much of a turn off I guess.
Anyway, an hour later and $400 lighter in the wallet, I was the proud owner of a faded, mildly rusty, powder blue Pinto Wagon riding on stock tiny wheels and tires and a very tired original suspension, with a fire breathing Dragon under the hood that was attached to what had to be the dead quietest muffler they could find.
Silent but deadly.
That car was outrageous. It could easily smoke the tires right off of the wheels.