Which is the strongest reducing agent?

Answer: In chemistry, a reducing agent (sometimes termed a reductant, reducer, or electron donor) is a chemical substance that “donates” an electron to an electron receiver (called the oxidising agent, oxidant, oxidizer, or electron acceptor). Regularly used reducing agents include earth metals, formic acid, oxalic acid, and sulphite compounds.

H₂S  is the strongest reducing agent. Since S is in its lowest oxidation state in H₂S, it has more electrons to spare, making it the most powerful reducing agent.