If by "best" one means the sign in which Mercury is most resonant ("best off"), everything else being equal, I would have to vote for Virgo (which I did) Mercury both is domiciled in the entire sign of Virgo, and exalted in certain degrees of that same sign (according to the ancients in all thi; also according to Vedic astrology)
Virgo is my own rising sign (by either tropical or sidereal calculations) and I can personally testify to the influence of Mercury through Virgo.
So I'll vote for Virgo being the "best sign" for Mercury.
(Note: Cancer not being very beneficial for Mercury might be due to the "natural inimicals" relationship between the Moon and Mercury: in traditional astrology and also in the Vedic system Moon and Mercury are considered "natural enemies", which I interpret energetically as being dissonant in their energy fields)
Virgo is my own rising sign (by either tropical or sidereal calculations) and I can personally testify to the influence of Mercury through Virgo.
So I'll vote for Virgo being the "best sign" for Mercury.
(Note: Cancer not being very beneficial for Mercury might be due to the "natural inimicals" relationship between the Moon and Mercury: in traditional astrology and also in the Vedic system Moon and Mercury are considered "natural enemies", which I interpret energetically as being dissonant in their energy fields)