Hiruzen wasn't a bad Hokage. He saw Konoha to the end of three wars and helped the village grow and prosper for, say, forty-ish years. His one mistake was letting Danzō deal with the dark matters within the village. He was afraid of getting his hands dirty or something? Of making the villagers hate him? Or it could be that he was too much of a softy. That was his flaw as a leader, it indirectly led to the Uchiha Massacre (more directly caused by Danzō), to Naruto's mistreatment (passively allowed by Danzo—who's "job" he unofficially made it to deal with dark matters—and therefore nothing was done), and to Orochimaru being at loose (who Danzō illegally fraternized with him, and then who killed him). Hiruzen wasn't the best Hokage in terms in internal affairs, but he wasn't a bad one all-around. Konoha would've buckled if not for him and his softness, it'd essentially be like ROOT, and whatever Will of Fire or whatever that he helped foster, that wouldn't exist anymore if he and his softness weren't there.
He died, was revived, admitted his faults, whatever. But he was fine. Maybe he wasn't the best, sure, but he wasn't the worse. Compared to all the other options available at the time of Tobirama's death, he was fit as a leader. He was the first one offer himself for the sake of the others, after all. And he survived longest, which says something. (Koharu and Homura were canonically weak and just plain lucky, and Danzō was screwed in the head, obviously.) Maybe he wasn't a good man, let bad things happen, didn't care for the children enough, etc… but as the Hokage of the Hidden Leaf, he did fine. More than less. All his faults were passive, not active; by omission, not commission. He wasn't perfect, but he wasn't a demon, he was a man. He tried to take after Hashirama, while Danzō clearly imitated Tobirama a bit more.
Tobirama wasn't a bad man, or maybe his actions are understandable considering how he'd been at war with the Uchiha Clan for thirty-ish years. He was just unfit to be leader. He wasn't good as a Hokage. His distrust for the Uchiha is what led to their end (belatedly proving Izuna and later Madara to be right about the danger in trying to trust the Senju, unfortunately, since Hashirama did originally have wholesome intentions). He established the Academy (nice), established the Chūnin Exams (uh-oh), established ANBU (nice), and established the Uchiha Police Force (bad). The Police Force was founded in order to keep an eye on the Uchiha, and it also served as an excuse for the laws in place that barred the Uchiha Clan from positions of influence (ANBU, Hokage, Council, etc). (*Itachi Shinden explains more in-depth.) Hashirama's two reasons for founding Konohagakūre along with Madara were: 1. To live peacefully with the Uchiha, 2. To make a place were kids would never have to fight again.
Tobirama backslid on both of these goals, clearly considering them to be too "idealist", as he was very pragmatic. I'd disagree, considering how the village itself was an idealist concept, but Hashirama and Madara made it work together.
Anyway, Madara most definitely was not unstable. He canonically loved Izuna more than Itachi loved Sasuke, but while Itachi was ready to spill Konoha all on the floor if anything happened to Sasuke, Madara set Izuna's death on the back-burner and was willing to work with the clan, the man who killed his brother, for the sake of the Uchiha Clan—which he promised to protect. He could've done that perfectly as Hokage, and Hashirama was all for it. Things would've gone well if Tobirama didn't interfere, honestly. But anyway, when Madara saw that it wasn't gonna work out because he'd heard Tobirama basically say just that, he tried to get them to leave with him, but they refused. The Uchiha Clan wanted Konoha, they wanted peace. To think they're unstable little red-eyed monsters waiting to explode was unjust and uncalled for, they loved the village just like everyone else. (Cue Madara feeling guilty since he'll be unable to fulfill his promise, coming back to wreck Konoha himself, losing, yadda yadda this ain't about him.)
Tobirama took Madara's body, studied his eyes a bit, then decided that the Uchiha were all emotional freaks and barred them from rising to anything in Konoha even despite Hashirama telling him not to. He made the Uchiha Police Force to help them "channel their emotions" based upon a blanket conclusion he made about the entire clan from studying one dead guy's body—a dead guy the clan didn't even agree with. That was inequitable.
As a Senju, that was pragmatic. As a ninja, it's smart to keep people you distrust at a distance. But as Hokage, he had a responsibility to view all the villagers as his people to protect, all of them, including the Uchiha Clan. And he didn't. He suppressed them instead, because he didn't trust them.
Which is natural for him as a Senju, but it's the exact reason why he shouldn't have been Hokage. Danzō caught on, his will was clearly carried on in him. This will and way of "protecting" the village at the expense of the dangerous Uchiha Clan, (who are a part of the village?). What Danzō's done is on his own back, and I won't be explaining them since this is about Tobirama, but it does all reflect back on him.
Tsunade wasn't a bad Hokage at all. A "bad Hokage" for not doing anything—? What is she supposed to do? Nothing's even happening during her reign, but when things do happen (e.g. Akatsuki's Invasion, Fourth Great Ninja War, etc) she does do stuff about it (e.g. Using her chakra to heal literally the entire village, going alongside all five great villages to ally against "Madara"/Obito, speaking up for Naruto!!, fighting—and getting creamed by (but that's beside the point)—Madara, etc.) She came for the very purpose of being the village's pillar after a month of having no Hokage, she came and immediately released Kakashi and Sasuke from a genjutsu placed by the mangekyou sharingan. She was great.
I'm rambling but it's been three years since this thread's been active so I'll take that liberty. Basically, Hiruzen was only bad at internal affairs but he was implicitly good at everything else (and things would've worked out between him/Shisui/Itachi/the Uchiha, if not for Danzo), Tobirama held prejudice and was unfit to be Hokage, he was a bad Hokage (and whatever he established about the village's infrastructure could've been established perfectly well from the position of adviser or something, things would've worked out nicely between Hashirama and Madara if he didn't interfere), and Tsunade was just an excellent Hokage who came to save the village from crumbling after its devastating attack from the Oto/Suna-nin and then from Akatsuki. What a queen,jeez.