What is the main spirit used in a Tom Collins?

The main spirit used in a Tom Collins is gin. This cocktail is a classic highball that combines gin with lemon juice, simple syrup, and soda water, served over ice in a tall glass. The distinctive botanical flavors of gin complement the tartness of lemon juice and the sweetness of the simple syrup, creating a refreshing drink that's popular in warm weather.

Choosing gin aligns with the traditional recipe for a Tom Collins, making it essential to the drink's identity. The use of other spirits like rum, whiskey, or vodka would not result in a classic Tom Collins, as each of those spirits offers a different flavor profile that alters the character of the cocktail significantly.