Much as I love the Apollo missions, if I had to nominate a NASA program that is my favourite, I would not hesitate to say the Voyagers 1 and 2. Launched in 1977 to coincide with an historic alignment of the four gas giants in our solar system, these intrepid and extraordinary craft not only achieved their primary goals of visiting said giants, but have gone on to possibly even greater things by flying further from Earth than any space probe before or since.

The rare  alignment of Neptune, Uranus, Saturn and Jupiter meant that both craft could get a fuel-free boost (gravity assist) from each of the planets to fling it onto its next task. The mission would not have been possible if not for this assistance.

Not only was some great science done (for example, we found out that Jupiter's  Io was an active moon with observable vulcanism) but both probes have now breached the heliosphere, the region surrounding our star, the Sun, beyond which interstellar space begins. Just stop and think for a minute what a remarkable thing that is!

I have posted a representation of the heliosphere breach before, but here is another, courtesy of NASA.