Question

which is the logest kappiyam in world

Answer 1

The longest kappiyam in the world is Mahabharata.
And the oldest surviving kappiyam is The Epic of Gilgamesh.
I don't know whether you asked the oldest or longest but I gave the answer for both! ^_^

Answer 2

The Mahabharata is the longest kappiyam in world. Maharshi Vedavyasa is considered as the one who wrote it.