IMHO, Cassini is one of the most successful space missions ever, crewed or not. Not only it managed to drop a lander a Titan (a very very remote and alien world when you think to it !), but it achieved countless flybys in the Saturnian system, giving a lot of clues understanding the ring system and the moons history. It also gave us awesome pictures of those previously very uninteresting pieces of rock and ice. Those Enceladus ice geysers proved us how complex and interesting small bodies can be, even far from the Sun.
To sum it up :