best antifreeze for old cars

Depends... if it is a basically stock rebuild, i.e. iron heads, intake, brass radiator- then the old reliable green ethylene glycol (IAT- inorganic acid technology) is still probably the best and most cost-effective way to go.
But the more aluminum you add to the system- aftermarket intake, aluminum heads, aluminum radiator; the more I would consider using a HOAT (hybrid organic acid technology) type anti-freeze (usually orange or yellow); they have silicate additives to help fight corrosion in the aluminum components. It has about a 5 year working life.
With just an aluminum radiator, you should be able to get by with green- as long as you change it out regularly.