The Los Angeles Times' Deborah Netburn just wrote a really nice retrospective; ‘OK. Let’s do it!’ An oral history of how NASA’s Cassini mission to Saturn came to be with plenty of photos and quotes from contributors to the spacecraft.
Some of the images really surprised me! Everyone views a spacecraft differently, but from a perspective of size, Cassini is two big things; a giant fuel tank to go far away from home, and a giant dish to stay in touch with home, and possibly to protect the spacecraft from direct impacts from ring particles during the more daring maneuvers.
This made me look up the wet and dry masses of Cassini. From the Wikipedia page for Cassini-Huygens I see a launch mass of 5,712 kg and a dry mass of 2,523 kg. I am not sure if these both include the Huygens lander or not.
If I just use those two numbers, I get about 56% of the mass as fuel, but I am not sure if this is the correct usage of the numbers because of how Huygens factors in, so I thought I would pose the question to the experts, and let you decide how to handle the math:
Question: Which deep-space spacecraft had the largest fuel mass fraction?
above: "Engineers and technicians at JPL work to complete the stacking assembly of the Cassini spacecraft in 1996." From here. Ken Lubas / Los Angeles Times
above: "Parts of the Cassini spacecraft are assembled at JPL in 1996." Cropped, from here. Iris Schneider / Los Angeles Times
above: "A diagram of the Cassini spacecraft and Huygens probe." From here. NASA Jet Propulsion Laboratory. Open in a new window for full size so you can see the detail and read the descriptions!