Which spacecraft flew past Jupiter and Saturn, taking crucial data about these gas giants?

The spacecraft that successfully flew past Jupiter and Saturn, providing significant data about these gas giants, are the Voyagers. Voyager 1 and Voyager 2 were launched in 1977 and conducted extensive studies of the atmospheres, moons, and rings of both planets during their flybys in 1979 and 1980, respectively. The data collected has vastly expanded our understanding of these distant worlds, revealing details about their composition, weather systems, and magnetic fields.

The Pioneers, while they did explore the outer solar system, were not equipped to collect the same level of detailed data and did not have the same extensive mission objectives as the Voyagers. Mariners and Lunar Orbiters focused on Mars and the Moon, respectively, and therefore did not study Jupiter or Saturn. Hence, the Voyagers are the correct answer for this question, as they provided the crucial data referenced.