5 if the water flow into 1 is small.
If the pipe from 5 into 2 is opened and the water flow into 1 is slow than the answer is 3. 2 will be almost full but not completely full. 6 won't fill all the way because 3 would overflow first.
Also if the water flow into 1 is much faster than the outflow to 5 than 1 could overflow first.