There were 2 books that inspired my love of Greco-Roman myth: one for me as a child and one as a teenager.

Best is subjectively, but the teenager in me loved Edith Hamilton's Mythology.

Me as a kid loved D'Aulaires' Book of Greek Myths.

Although, I would definitely read The Illiad or The Odyssey as well. The poetical asides in each Homerian work allude to myths, like referencing a political event on a tv show. And since they're so integral to the mythological canon, I would prefer to start there. But the books I referenced are good to whet the appetite.