John 1:18

No man has seen God at any time; the only begotten, being what God is, who is in the bosom of the Father, he has declared him.

θεὸν οὐδεὶς ἑώρακεν πώποτε· μονογενὴς θεὸς ὁ ὢν εἰς τὸν κόλπον τοῦ πατρὸς ἐκεῖνος ἐξηγήσατο.

Greek

Compare with the ASV

No man hath seen God at any time; the only begotten Son, who is in the bosom of the Father, he hath declared him.

What we did

We followed the SBLGNT reading μονογενὴς θεός and rendered it "the only begotten, being what God is," in place of the ASV's "the only begotten Son."

Why

The manuscripts
μονογενὴς θεός is the reading of P66 and P75 (the earliest papyri, c.
More
AD 200), Codex Sinaiticus and Codex Vaticanus, and is printed by NA28 and SBLGNT. ὁ μονογενὴς υἱός, "the only begotten Son," is the reading of Codex Alexandrinus, the Byzantine majority, the Textus Receptus, and Old Latin and Syriac witnesses; the ASV and KJV print it. It matches John's phrase in 3:16, 3:18 and 1 John 4:9.
The base text
This version follows the critical Greek text throughout (for example 2 Thess 2:3), so it follows it here.
The article
θεός in "μονογενὴς θεός" has no article, as in 1:1c, and is rendered the same way, qualitatively: "what God was, the Word was" there, "being what God is" here.
"Declared."
ἐξηγήσατο means "explained, made known." The ASV's "declared" is kept.

Other renderings

"the only begotten Son" (ASV, KJV); "the only begotten God" (NASB); "the unique one, who is God" (NET).