Which car was rare to begin with and keeps increase noticeably in value every few years? The 250 GTO was a street-legal race and championship-winning sportscar and production was limited to only 39 cars.
Is it a muscle car like the '70-'71 Plymouth Hemi 'Cuda convertible, only 14 were made in '70 and 7 made in '71, cars that are pulling at least 1 or 2 million at auctions? They're one of the unicorns of the muscle car era, the last of the Hemi at its most powerful iteration tucked into an aggressive and an iconic Mopar body. However, while the performance numbers were impressive the 'Cuda never really won any major races or series and the Hemi was known to be very unreliable on the street.
Or maybe something more Italian like the Ferrari F40? 1,315 officially produced, many raced and/or wrecked yet, some pristine examples are going for over 1.5 million dollars. The F40 was a pinnacle of late-80s supercar performance, even holding the title of fastest production car for a short time at 201 mph (not 200, no, we go 201). Like the 'Cuda, the F40 wasn't dominant in the racing world. However, an F40 did win in the BPR Global GT Series at the 4 Hours of Vallelunga in '94 and the 4 Hours of Anderstorp in '95 and '96.
In my opinion, the next 250 GTO is the McLaren F1, and all of its iterations.
Why? Because it dominated, on and off the track. It won LeMans overall in 1995 and taking first in GT1 in 1997 second overall, it dominated the BPR Global GT taking 2 out of 2 championships (the series lasted from '94 to '96 but no champions were crowned in '94), and it earned the special honors of being the only non-Japanese car to win races in the Japanese Grand Touring Championship's top class and to take the overall championship itself!
It was the poster-boy for supercars for nearly ten years starting in 1992 until the Bugatti Veryon and the Koenigsegg CCR/CCX took performance to the next level. It had one of the most powerful naturally-aspirated production engines of its time, the driver sits in the middle like a formula car, it went a record-setting 240 mph, and it had no safety nannies to keep it in check, the driver was the only one who truly had control. Clarkson complained the "normal" F1 was twitchy and all production F1's were limited to 225 for stability reasons but no one cared because they had a car with the potential to hit 240 if they were crazy enough to attempt it.
Finally, it has rarity, only 108 chassis were produced and a good dozen of them were wiped out by crashes on the street and the track. There're 6 LeMans celebration editions, 3 long-tails, and the rest are a mix of normal and racing chassis. F1's originally went for a cool one million dollars new but Michael Andretti's personal F1 fetched ten million dollars at a recent auction. Expect prices to keep rising as more and more people desire to have a piece of Superman's cape.