Taken literally, a choice wine is a wine that you would choose above all others. In a Bahá’í context, the phrase “choice wine” is probably best known from its use near the beginning of the Kitáb-i-Aqdas, where Bahá’u’lláh issues this warning:
Think not that We have revealed unto you a mere code of laws. Nay, rather, We have unsealed the choice Wine with the fingers of might and power. To this beareth witness that which the Pen of Revelation hath revealed. Meditate upon this, O men of insight!
In the Epistle to the Son of the Wolf, Bahá’u’lláh uses this same metaphor in an admonition to the Epistle’s recipient, Shaykh Muhammad Taqi:
O Shaykh! Verily I say, the seal of the Choice Wine hath, in the name of Him Who is the Self-Subsisting, been broken; withhold not thyself therefrom.
Volume 4 of the book “The Revelation of Bahá’u’lláh” gives an explanation of this metaphor, which I’ll quote here:
‘The seal of the Choice Wine hath…been broken.’ The reference to choice wine, sealed, may be found in the Qur’án (see 83:22-6). The significance of ‘sealed’ is that the true meanings of the Words of God in former Dispensations were not disclosed until the advent of Bahá’u’lláh (see vol. 1, pp. 160-61, on this prophecy of Daniel). The unsealing of the Choice Wine signifies the revelation of the Word of God in this age, disclosing to mankind new teachings and new laws. Bahá’u’lláh declares in the Kitáb-i-Aqdas, ‘Think not that We have revealed unto you a mere code of laws. Nay rather We have unsealed the Choice Wine with the fingers of might and power.’ [The Kitáb-i-Aqdas, ¶5]
So, tl;dr: It’s not literally wine, but a figurative wine that symbolizes the Word of God for this age, which Bahá’u’lláh has revealed.