Bg 18.64

sarva-guhyatamam bhuyah
srnu me paramam vacah
isto ’si me drdham iti
tato vaksyami te hitam

Word for word: 
sarva-guhya-tamam — the most confidential of all; bhuyah — again; srnu — just hear; me — from Me; paramam — the supreme; vacah — instruction; istah asi — you are dear; me — to Me; drdham — very; iti — thus; tatah — therefore; vaksyami — I am speaking; te — for your; hitam — benefit.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Because you are My very dear friend, I am speaking to you My supreme instruction, the most confidential knowledge of all. Hear this from Me, for it is for your benefit.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Lord has given Arjuna knowledge that is confidential (knowledge of Brahman) and still more confidential (knowledge of the Supersoul within everyone’s heart), and now He is giving the most confidential part of knowledge: just surrender unto the Supreme Personality of Godhead. At the end of the Ninth Chapter He has said, man-manah: “Just always think of Me.” The same instruction is repeated here to stress the essence of the teachings of Bhagavad-gita. This essence is not understood by a common man, but by one who is actually very dear to Krishna, a pure devotee of Krishna. This is the most important instruction in all Vedic literature. What Krishna is saying in this connection is the most essential part of knowledge, and it should be carried out not only by Arjuna but by all living entities.