Before we get down to the individual spirits – and I’m using the term spirits to include liqueurs and vermouths which isn’t technically correct but eases navigation – it’s a good idea to have a look at the construction of a typical cocktail. An awful lot of the time a cocktail has three components (remember everything comes in threes in cocktailworld). The following is the old school dissection of the cocktail, but I still think it makes the most sense.
A base spirit. The classic cocktail bases of old were whisky, cognac, gin, and rum. Post 1945 we can add vodka and tequila to the list and, more recently, mezcal. Other bases remain pretty much on the fringes. The base spirit accounts for the majority of the volume of a cocktail. Usually. Because of this the quality of the base spirit is essential. A low quality base will not be significantly masked by the following.
A modifier. The modifier adds an extra dimension to the base – eg. citrus, vegetal, spicy – as well as a sweet, sour or bitter element. Examples would be vermouths, amari or fresh fruit juice.
An accent. The accent has the tricky task of bringing the cocktail into balance and, optionally, adding a hint of another flavour to the mix. An example might be using a vanilla syrup to bring balance to the sour of a Daiquiri while adding a slight twist. Bitters can be used as either the main accent or an add-on second accent. Syrups and liqueurs are typical accent ingredients.
These components are often (but not, of course, always) used in the proportions of 8 parts base, 3 parts modifier, 2 parts accent, with a part being 0.25oz. So 2oz rum, 0.75oz lime juice, 0.5oz sugar syrup would be typical. Bars will often cut the 2oz back to 1.5oz for economic reasons. A more basic version of this – 2:1:1 – is more commonly quoted these days but is likely to lead to an overly sweet drink, unless the “1” is composed of drier liqueurs which are less sweet than a syrup.
Remember, rules are meant to be broken, and the all of the above should be viewed as a starting point to understanding the way flavours play out in a cocktail. Fine tuning by tasting is essential.