Just about every cocktail needs something added to give it taste. Sometimes you need to mask the taste of the booze, sometimes you need to enhance the taste of the booze.
These are probably the most famous and recognizable bitters around. Originally created as a tonic and a digestive, these bitters are now a very popular way to add flavor to a cocktail. They also are 44.7% abv.
Probably the second most recognizable bitters, Peychaud's is a staple ingredient in the New Orleans classic cocktail the Sazerac.
Angostura Orange Bitters
Made with orange peels grown in Trinidad, Angostura has been making these orange bitters since 2007. They are a sweeter more versatile bitter, yet still have an abv of 28%.