Tom Collins
Jack Byrnes (Robert De Niro)
Jack's go-to drink is the Tom Collins, which comes up repeatedly in the film.
Sources: Tasting Table: 12 Iconic Times Gin Cocktails Made It To The Big Screen; VinePair: Drinking With the Stars: 11 Cocktails and the Movies and TV Shows That Made Them Famous; Cheryl Charming (Miss Charming): Cocktails in Film