best man
best man is acceptable only if his partner is "best woman." Otherwise use attendant, attendant of honor, honor/wedding/groom's attendant, with a parallel term for the female attendant. Male and female wedding attendants did not originally have the same duties, which is perhaps why nonparallel terms developed. Centuries ago, in some cultures, men enlisted the help of their friends to kidnap a future wife or to prevent her from being kidnapped by someone else.
