The consensus is, unless you want to be a specialist, it is best to stick to a basic range of cocktails that won't need much training of staff nor any big investment in strange ingredients and rarely-used equipment.

Six classic cocktails every bar should be able to do...

Harvey Wallbanger
50ml vodka, orange juice, dash Galliano
Add the vodka and orange juice to a highball glass filled with ice. Float the Galliano onto the surface of the drink. Garnish with an orange wheel and serve with two straws.

Cuba Libre
50ml white rum, 4 lime wedges, cola
Add a measure of white rum to a highball glass filled with ice. Squeeze the lime wedges and drop the husks into the glass. Top with cola, stir gently and serve with two straws.

Bloody Mary
50ml vodka, 2 large dashes tabasco sauce, 2 large dashes Worcestershire sauce, 1 pinch black pepper, 1 pinch
celery salt, tomato juice
Add all ingredients to a highball glass filled with ice. Stir gently, garnish with a lemon slice and serve with two straws.

Tequila Sunrise
50ml gold tequila, orange juice, dash Grenadine
Add the tequila and the orange juice to a highball glass filled with ice. Stir gently then drizzle the grenadine down the inside of the glass so it rests on the bottom. Garnish with an orange wheel and serve with two straws.

Long Island Iced Tea
5ml vodka, 5ml gin, 5ml Cointreau, 5ml tequila, 5ml white rum, 25ml lemon juice, cola
Add all ingredients (except the cola) to a shaker filled with ice. Shake sharply and strain into a highball glass filled with ice.
Top with cola, stir gently, garnish with two lemon slices and serve with two straws.

Moscow Mule
50ml vodka, 2 lime wedges, Ginger beer
Add a measure of vodka to a highball glass filled with ice. Squeeze the wedges and drop the husks into the glass. Top with ginger beer, stir gently and serve with two straws.

Six more that ought to be on any decent list...

Margarita
50ml tequila, 20ml Curacao liqueur, 20ml lime juice
Add all the ingredients in a cocktail shaker and shake with ice, then strain into a chilled glass rimmed (or half-rimmed) with salt, and decorated with a lime wedge.

Manhattan
50ml rye whiskey, 25ml sweet vermouth, 2 dashes Angostura Bitters
Pour all the ingredients over ice in a mixing glass and stir. Strain into a chilled martini glass. Garnish with a cherry.

Black Russian
50ml vodka, 25ml Kahlua, cola
Add all ingredients to a rocks glass filled with ice. Stir gently, garnish with an orange zest and serve with two short straws.

Daiquiri
50ml white rum, 25ml lime juice, 25ml sugar syrup
Shake all the ingredients with ice and strain into a chilled cocktail glass.

Tom Collins
50ml gin, 25ml fresh lemon juice,
12.5ml sugar syrup, soda water
Add all ingredients to a highball glass filled with ice. Stir gently, garnish with a lemon slice and serve with two straws.

Slow Comfortable Screw
25ml sloe gin, 25ml Southern Comfort, 120ml orange juice, 1 orange slice
Build in a highball glass over ice. Garnish with the orange slice.

And one more for the adventurous...

Caipirinha
50ml Cachaca, 20ml sugar syrup, 1/2 lime (quartered)
Chill a rocks glass with cracked ice. Place the lime quarters in the bottom of a mixing glass, add the syrup and muddle. Add the cachaca to the mixture in the mixing glass and shake well. Pour the entire contents of the mixing glass back into the chilled rocks glass and serve.