In the Naruto series, only Second Hokage Tobirama and Third Hokage Hiruzen are often discussed as the worst Hokages.
Among the seven Hokages of Konoha, two are known to have caused many unfavorable events in the Naruto story. They are Second Hokage Tobirama and Third Hokage Hiruzen.
But between Tobirama and Hiruzen, who is the worse Hokage?
1. Contributions to the Leaf Village and the Mistakes of Second Hokage Tobirama

Second Hokage Senju Tobirama made significant contributions to the development of Konoha's organizational system, as well as establishing fundamental structures for the village such as building the ninja academy, forming the Anbu organization, the Konoha guard force, and the Chunin exams.
Despite his many achievements, Tobirama still made regrettable mistakes.

Tobirama grew up in a time of war, hailing from the Senju clan which had an age-old feud with the Uchiha. Thus, he harbored conflicts with the Uchiha from a young age. This led Tobirama to become distrustful and resentful towards the Uchiha clan, resulting in regrettable behavior later on.
The Konoha guard force was established to showcase the Hokage's trust in the Uchiha clan, so most of its members are from the Uchiha clan. However, Tobirama never truly trusted the Uchiha clan. His true intention in forming the guard force was to isolate and segregate the Uchiha from the village, not forgetting to deploy the Anbu team for strict surveillance.
In addition to his mistakes with the Uchiha clan, Tobirama also created a dangerously forbidden technique, threatening human lives.
It's undeniable that Tobirama is an extremely talented ninja. He excelled not only in ninjutsu but also as a pioneer in creating many powerful techniques through his intellect, such as Shadow Clone, Teleportation Jutsu, and even Edo Tensei (Reanimation Jutsu).

The Edo Tensei technique revives the deceased by sacrificing the living. The question remains: How did Tobirama create this technique? Did he use prisoners of war for his experiments? This is still a mystery.
However, later on, Orochimaru exploited this forbidden jutsu after perfecting it. He once resurrected Tobirama and Hashirama themselves, leading to the departure of the third Hokage, Hiruzen. Moreover, Kabuto also used Edo Tensei to summon countless powerful shinobi of the past, who had been lost, to oppose the ninja alliance in the Fourth Great Ninja War, causing numerous casualties.
2. Contributions to the Leaf Village and the Mistakes of the Third Hokage, Hiruzen

In his governance of the Leaf Village, Hiruzen embraced the belief that all Konoha residents were his family. Hence, he often interacted with and sincerely treated the villagers.
However, this is just the tip of the iceberg; in the shadows, Hiruzen still committed clandestine acts. Allowing Danzo to operate unchecked caused many troubles and conflicts in the future.

However, Tobirama or Hiruzen, who is the worst Hokage in Naruto?
Even Hiruzen couldn't decide and find good solutions for some conflicts.

Although Danzo was the mastermind behind the Uchiha clan massacre, Hiruzen's mistake was being too slow in making decisions, allowing the regrettable incident with the Uchiha clan to occur.
When discovering signs of Uchiha rebellion to take control of the Leaf Village, Danzo assigned Uchiha Itachi to spy on the clan in hopes of gathering information. At that time, Hiruzen only promised to find a peaceful solution for both sides, but when the coup was imminent, Hiruzen had no solution. Consequently, Danzo ordered Itachi to annihilate the entire Uchiha clan and leave the village, carrying the blame for massacring the clan and betraying the Leaf Village.
A similar incident is the conflict between the Hyuga clan and the Hidden Cloud Village - the mastermind behind the plot to kidnap Hinata for personal gain. As Hiruzen couldn't provide a good solution to this conflict, ultimately Hizashi - Neji's father, became a victim to reconcile this conflict.
Furthermore, Hiruzen also seemed to handle his former student Orochimaru with a bit of leniency, including turning a blind eye to his shady activities, causing many to become victims of Orochimaru's experiments.
7. Conclusion, Hiruzen is a worse Hokage than Tobirama

Undeniably, Hiruzen is indeed a friendly and caring Hokage, but unfortunately, many of his decisions were misguided, leading to the sacrifice of many. During his time, the issue of bullying persisted in Konoha, even towards renowned ninjas like Sakumo, or the cruel mistreatment of the boy Naruto. Allowing Danzo to act as he pleased and making decisions or solutions too late resulted in numerous conflicts and losses.
As for Tobirama, although he was steadfast and strict, some of his decisions and innovations also led to regrettable outcomes. However, they didn't result in as many deaths as what Hiruzen had done.