Which classic car should I buy?

1.68-70 Dodge Charger
2.68-70 Plymouth Road Runner
3.67-70 Ford Mustang

Added (1). Resto-Mod. Restore the car to a working point, but not not factory spec. Just enough.

All models are great choices but then we must look at how values have held up etc. Mustang took center stage being the 1st 60's car to become a hot commodity. The 65 mustang began to be seriously sought after. This however depended on the trim package. The 289 engine, closely followed by the 260 engine based on the same V8 design but earlier production. The 4 speed was slightly more liked than the automatic and convertible a big plus. Shelby mustangs were always collectable prior to this. They sported the hi- po 289 till 67 and this too could be found in the 65 models. Pony interior package was also a big plus. Ford introduced more power in the 67 models up offering 289, 302, boss 302 390, 390 hi-po. 427 (4 and 8 barrel) 428 and cobra jet 428. 70 offered an additional boss 429. Basically you went by horsepower to see costs to buy. Notably the 6 cylinder models were not mentioned as they were nearly not collectable on the auction floor.
Roadrunner shared the same platform basically as the Dodge with skin styling different. The engines and drivetrain were the same options. The base 318 ci, the 340, 360, 383 440 and 426 hemi. The 340 on had horsepower options (dual quad, higher performance heads and exhaust systems as well as gear ratios for drag racing) Interiors from plain Jane stripped dash, to plush carpeting and accessories. Both Dodge and Plymouth had specialty models of these cars (similar to how Shelby worked the mustangs) in the Daytona Charger and the Plymouth Super birds. They also had super bee in the line up some noteworthy cars not mentioned and more rare Big block darts and high performance Duster demons. Both were equipped from the factory with high performance engines and drive trains.
Ford as well had some special models out there. Mercury Cyclones with 429 boss power, boss 302 cougar eliminators and 428 cobra jet versions as well. The Torino match up to the super bird with a 428 super cobra jet motor. All have collectability.

All are great! I have a 67 mustang and have since I was 16, so roughly 11 years. You didn't say if you were going for a project car but I think it was a great car to learn mechanical skill on. Good luck with your choice. I love the others too. What are you looking for in your car? A "complete" car or something running but a project to work on?

They all have their pros and cons. But considering those vehicles are now at least 45 years old, the pickings of good examples is slim unless you have big bucks to spend.

Be prepared to pay a lot of money upfront or a lot of money restoring them ( assuming a restoration is something you want to do ).

69 mustang. It stills view

MOPAR OR NO CAR! Any of those you have mentioned are great

100% Buy the Road Runner. That isn't even a debate.