Tsunade is a great hokage, and in all honesty, you can argue that she was one of, if not the best. Now before people flame me for that bold statement, NOTE THAT I AM NOT TALKING ABOUT POWER.
Based on policy, she is definitely better than Hiruzen and Tobirama, and she busted her ass to save the Leaf, just like Minato and Hashirama. She might not have fought pain, but she did a damn good job saving everybody that was alive after Pains initial assault from his Almighty Push jutsu that destroyed the leaf.
She believed in Naruto and didn't keep him cooped up, like the elders and Danzo tried to do, and she actually cared about Naruto (like a grandson), which you could reason that Hiruzen didn't. In fact, the only thing Hiruzen ever did for Naruto is get him an apartment and let him enter the ninja, he turned a blind eye to the way that the villagers tortured Naruto. Not only did she allow Naruto to push himself, she allowed the rest of the K11 to push themselves too (like Shikimaru) and she even saved Lee's life as well.
She is easily top 3 in all around hokage rankings (not just power), and it can be argued that she surpassed Hashirama due to the fact that she is one of the 5 kage that banded their nations together to try to change the world (thus accomplishing Hashirama's dream). She is a great hokage.
