long suit

1. Games. The suit in which a player holds the most cards in a given hand.
2. The personal quality or talent that is one's strongest asset.

noun

Something at which a person excels: forte, métier, specialty, strong point, strong suit. Slang bag, thing. See ability/inability.

One's strong point or advantage, as in Organizing has never been Nancy's long suit. This expression alludes to whist, bridge, and other card games in which holding numerous cards in a single suit may convey a strong advantage. [c. 1900]

The noun has 2 meanings:

Meaning #1: in a hand, the suit having the most cards

Meaning #2: an asset of special worth or utility
  Synonyms: forte, strong suit, metier, specialty, speciality, strong point, strength