It was a bartender friend of mine who tut-tutted that my well-stocked home bar was missing something crucial: bitters. Production problems have led to a global shortage of the best known brand, Angostura, but it turns out that there are many and various alternatives whose herby, complex flavours add a reassuringly adult twist to many drinks.
Now, after lengthy late-night experiments, I'd suggest three bitters for any home bar: Peychaud's, which was used in what is thought to be the first ever American cocktail, the Sazerac (see http://en.wikipedia.org/wiki/Sazerac for the full recipe); some orange bitters (I like Regans' Orange No 6); and some Bitter Truth Old Time Aromatic.
I'd recommend trying one cocktail in particular for proof of why I’m a bitters believer: the fitty-fitty (fourth picture). The name is a nod to the recipe (fifty-fifty), an ounce each of vermouth and gin (I use Hendrick's), stirred in a shaker with three or four dashes of Regans' No 6. Fresh and crisp, it's like a fruity martini. The only downside is how easy it is to drink.