Rye or bourbon, sweet vermouth, and Angostura bitters — one of the five greatest cocktails ever created.
The Manhattan's origin is disputed — the most popular story attributes it to a bartender at the Manhattan Club in New York City in the 1870s, possibly for a banquet hosted by Jennie Jerome (Winston Churchill's mother). The drink appeared in Harry Johnson's 1882 'New and Improved Bartender's Manual' in a recognisable form.
Never shake a Manhattan. This is one of the non-negotiable rules of cocktail making. Shaking aerates the drink, creating a cloudy, diluted result. A stirred Manhattan is crystal clear, silky, and integrated. The vermouth is not optional decoration — use 1 oz of good vermouth. Carpano Antica Formula is the industry standard for a reason: its vanilla-walnut complexity makes a better Manhattan than any other sweet vermouth.
Replace rye or bourbon with blended Scotch. Named after the Scottish folk hero. The peat and earth of Scotch adds a completely different dimension.
Use ½ oz sweet vermouth and ½ oz dry vermouth. Drier, more complex, and slightly less approachable. Sometimes called the 'Uptown Manhattan'.
Replace Angostura bitters with Averna Amaro. Darker, more herbal, and with a bittersweet cola note.