Bucket 3. If you look closely, some buckets have no openings. Bucket 3 does not open into 4, and the pipe to 5 is also closed off. So it would fill up first. Then it would over flow and the second bucket filled would be bucket 4 followed by 5&6, 7, 2, and 1 (assuming all the buckets are in a room with a watertight floor and walls.