Russia Today has a story looking back at the history of the Soyuz-U rocket, on the occasion of its final launch. According to the story, the Soyuz-U has flown either 788 or 789 times, the first mission taking place in 1973 when it was used to loft "a Soviet military surveillance satellite" and the most recent carrying
[...] the Progress MS-05, a freighter carrying 2.5 tons of miscellaneous cargo – fuel, food, water, sanitation, oxygen and air, as well as spare parts, supplies and experiment hardware – to the International Space Station (ISS).
The story calls it "the longest-serving rocket in history."
Additional coverage:
(Score: 3, Insightful) by bob_super on Friday February 24 2017, @12:20AM
Now you've got me thinking about App-like Agile Sprint-based rocket development: "Sure, version 0.9.4 released the satellite from second stage before igniting the engine, but we clearly fixed the high-priority bug that had blown the previous one at 50km AND it was 25% closer to the expected flight path. Can we get your honest feedback on how much cooler the new tail fins and logo look?"