Oldest or earliest? That video claims to be the latter and is from Lenin's funeral in 1924. Stalin himself obviously survived Lenin by quite a few years and had many more opportunities to be filmed. The last occasion that he was captured by the camera, that I'm aware of, was at the 19th Party Congress in 1952, six months before his death.

(Note the near-robotic applause in that video. The 1952 Congress was perhaps the apogee of late Stalinism, being dominated by talk of the succession and the, removed, figure of Stalin himself.)