About Mid-Kage Level. He was very skilled in Suiton, probably could destroy an entire village with a Water Jutsu.
...but Anyways, Nagato was always stronger.
Yahiko knew this so he tried to keep Nagao alive at all costs.
Nagato was the team's trump card.
Just like how the Leaf Village has a Hokage, but the strongest is Naruto.
When the Hokage was selected.. Tsunade was the strongest.
Nagato became "god" after Yahiko's death. Before that Yahiko was the one everyone feared. =D
So you can't say that at that point Nagato was stronger then Yahiko.