At first, Hashirama's. Sage Mode does the same thing for everyone: it amplifies their base power using natural energy, and grants sensory powers and and enhanced reflexes. The Natural Energy one can draw in is relative to one's own Chakra levels; therefore, Hashirama's Sage Mode is strongest.
However, Naruto's current Sage Mode is not just a simple Sage Mode; it's a type of advanced Chakra Mode that grants Senjutsu on a level similar to a Juubi Jinchuuriki. The exact mechanism of this is unclear; Naruto seems to only have the Will of the various Bijuu, and not much Chakra from them, despite having power far exceeding his Kyuubi Mode. I think Naruto'z base power is now on par with Hashirama's Sage Mode, and Naruto's "Sage Chakra Mode" is 10x more powerful in terms of raw power and Chakra levels.