American · Cocktail
Two parts bourbon, one part sweet vermouth, two dashes of bitters, stirred cold and served up. Two ounces to one ounce is the ratio, and it hasn't needed changing.

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The one thing that matters
A good dark cherry, not the neon kind. Swap the sweet vermouth for dry and it's a Dry Manhattan; use half of each and it's a Perfect Manhattan, which is the only time that word belongs anywhere near a recipe.
It's older than almost every cocktail people think of as classic, and it survives because the proportions are right. Some people prefer it two-to-one the other way in winter. Nobody is wrong about this.
Rye makes it drier and sharper; bourbon makes it rounder and sweeter. Maker's Mark is wheated rather than rye-heavy, so it leans soft and slightly sweet, which is why it suits people who find a rye Manhattan too austere.
The vermouth is not filler, and it's where most Manhattans die. It's wine. It oxidises. Buy a fresh bottle, keep it in the fridge once it's open, and throw it out after a month. A Manhattan made with vermouth that's been sitting in a warm cupboard since last winter tastes flat and stale, and the whiskey gets blamed for it every time.
Stir for thirty seconds with plenty of ice. Shaking bruises it and makes it cloudy, and this is a drink you're meant to be able to see through.
Serve it up in a chilled coupe, not over ice. It's a small drink and it's supposed to be finished before it warms.