Where is the largest rift on Earth ?

Africa.

The Great Rift Valley is a name given to the continuous geographic trench, approximately 6,000 kilometres (3,700 mi) in length, that runs from Lebanon's Beqaa Valley in Asia to Mozambique in South Eastern Africa.The name continues in some usages, although it is today considered geologically imprecise as it combines features that are today regarded as separate, although related, rift and fault systems.

Today, the term is most often used to refer to the valley of the East African Rift, the divergent plate boundary which extends from the Afar Triple Junction southward across eastern Africa, and is in the process of splitting the African Plate into two new separate plates. Geologists generally refer to these incipient plates as the Nubian Plate and the Somali Plate.