The Master Bolt.

The Master Bolt is the symbol of Zeus' power, which all other lightning bolts are patterned after. It is the first and most powerful weapon that the Elder Cyclopes made for the Olympian gods. It is apparently so powerful that when Zeus hurled it at Typhon, the blast was said to be enough to have made entire oceans evaporate, the sky to be set ablaze and even knock the Father of all Monsters off balance.

During the Titanomachy Zeus and his brothers freed the Cyclopes that were imprisoned by his father Cronus. Zeus managed to talk to the Cyclops Brontes, and convinced him to forge powerful weapons for him and his siblings which would be more powerful than Cronus' Scythe.

Together with his brothers Steropes and Arges, Brontes forged three incredibly powerful weapons, the first of which was the Master Bolt, which was given to Zeus. The power of the Bolt made Zeus' body tingle with energy, and he tested his new mighty weapon on a huge nearby bolder, which was instantly blasted to dust. The bolt itself is considered to be the most powerful of the three weapons forged by Arges, Brontes, and Steropes, and all of Zeus' later thunderbolts were molded after it.

The Master Bolt is a two-foot-long cylinder of high-grade Celestial Bronze capped on both ends with god-level electrical explosives. It is stupendously powerful, "makes mortal hydrogen bombs look like firecrackers" according to Dionysus, with its power making Zeus' body tingle with energy when he wields it. When the Master Bolt is activated, the ends of the cylinder emit crystallized electric bolts, making it look like a classical lightning bolt. It can emit many tremendous bolts of lightning simultaneously. 

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