The best sword in the first sea of Blox Fruits is the Saber. It is considered the most powerful legendary weapon in the first sea, offering reliable performance against enemies, especially when upgraded. The Saber is known for its speed and high damage output, making it a popular choice for both PVP and grinding purposes. Additionally, the Soul Cane is suggested as an alternative to the Saber, available for purchase from the Living Skeleton NPC at Magma Village.