Hey, Classicist here. So one of the major problems when looking back at these earlier pantheons is by bringing our modern preconceptions and our Abrahamic faiths' biases into understanding them. Ancient Religion was a very loud, violent, and erotic affair just like their gods. Gods were a combination of natural factors and powers combined with very innate human drives.

In the mythos, Zeus is the highest because he A) led the overthrow of his and the other first five Olympians' father, Kronus(who killed his father Ouranos) and B) got lucky when he and his brothers picked lots on who would get what kingdom. Greeks really aren't fans of kinslaying if you would know from reading pretty much any Greek play and Zeus wasn't even the oldest so it's not like he inherited it. So what was it about the character of Zeus that led Greeks to put him on top? Well, he was the ultimate judge and protector. He is a close as the Greek gods got to Omniscience. Through several of his epithets, he can be said to be the god of rain, darkness, thunder, rage, winds, war, kings, property, the courtyard, justice, councils, marriage, fate, signs, orders, lots, wealth, hospitality, people, cities, of punishing murders and so much more. Zeus is an arbiter, a bringer of rains and justice. Nearly all of our sources are from much later in the record, so it is impossible to say with certainty what the reasoning of the inventors of this Zeus (or proto-zeus)had in mind, but we do know that to the people of Greece in constant contention with each other and with nature. A bringer of victory, the rains, and justice would sound incredibly appealing.

And to respond to your remark about the wimpiness of Zeus's lightning I give you a taste of what the Greeks felt about to the tune of Orphic Hymm 19

"To Zeus Keraunos (Ceraunus) (Thundering). O father Zeus, who shakest with fiery light the world, deep-sounding from thy lofty height. From thee proceeds the ethereal lightning's blaze, flashing around intolerable rays. Thy sacred thunders shake the blest abodes, the shining regions of the immortal Gods. Thy power divine the flaming lightning shrouds with dark investiture in fluid clouds. 'Tis thine to brandish thunders strong and dire, to scatter storms, and dreadful darts of fire; with roaring flames involving all around, and bolts of thunder of tremendous sound. Thy rapid dart can raise the hair upright, and shake the heart of man with wild affright. Sudden, unconquered, holy, thundering God, with noise unbounded flying all abroad; with all-devouring force, entire and strong, horrid, untamed, thou rollest the flames along. Rapid, ethereal bolt, descending fire, the earth, all-parent, trembles at thine ire; the sea all-shining, and each beast, that hears the sound terrific, with dread horror fears: when nature's face is bright with flashing fire, and in the heavens resound they thunders dire. They thunders white the azure garments tear, and burst the veil of all-surrounding air. O Zeus, all-blessed, may thy wrath severe, hurled in the bosom of the deep appear, and on the tops of mountains be revealed, for thy strong arm is not from us concealed. Propitious to these sacred rites incline, and to thy suppliants grant a life divine, add royal health, and gentle peace beside, with upright reasoning for a constant guide."