I try not to own anything that is decidedly not fun, so this is a hard question!
Small car: 1988 Mercury Tracer wagon.(in other words, a Mazda 323) Had a "meep meep" horn, 1.6/5 speed. Car weighed 2300lbs. Was geared to top out about 90mph. Was really fun and surprised people with how snappy it was up to about 60mph. Actually really comfy, and I am a giant. Thing ran forever too, and I beat it every day. Sold, 196,000 miles to my buddy. Original basically everything, the alternator pulley was so worn out it would eat belts lol. But this was years ago a spare belt was $6 and could be changed out with a vice grips if need be, I kept one belt and the one metric wrench in the glovebox. Rowing through the 5 speed pushing it to redline never got old. Sometimes driving a slower car fast can be really fun
I think officially 1/4 mile is listed at like 18.5 seconds, but estimations online about HP and 0-60 are frankly off, based on my 7 years of ownership. I suspect the 1/4 time is close though, it ran out of steam at about 70mph and slowly kept creeping up to about 95 at redline on the interstate.
Truck: Can't pick. 1991 Bronco(351W, long gone) my current Ramcharger and my '02 8.1/Allison 2500HD all have their own quirks and fun about them. Probably got into the most mischief in the '79 F250 farm truck in my youth though.
Sporty cars and otherwise: I really enjoyed my super modified '94 Mustang, but I think the most fun was a 351W swapped 84 Mercury Capri. man that thing was fast, burnouts from a roll in second gear, factory 85mph speedo. Poor, poor 85mph speedo. I don't know how fast 45mph is, the second time, but it seemed pretty f'ing fast lol. My '13 AWD Hemi Charger is really nice, but I don't see it as the "most fun" but it basically checks all the boxes of what I would want out of a family sedan: roomy, rides nice, good radio, goes like hell, drives through a foot of snow if I need it to, etc. but fun factor has to go to the pair of Mustangs.
If I had to pick one: the Capri. I think a car like that you set a timer on when it will "get you" though so I am glad it left my hands before the timer went off lol