Assuming that the water flow is enough to stop filling a container once a pipe appears, 1 is out. 3 is blocked off, which removes 3, as well as 6 and 7. 2 also never gets full due to the same as 1. The pipe to 5 obeys gravity, so 4 is excluded. That means 5 is the first!
However, if the water flow is too fast, 1 may fill up first.
