The aromatic version of Angostura Bitters is far and away the most common variety of bitters in the world. Almost every bar has a bottle on the counter and it is the only brand that has been widely commercially available throughout the 20th century (many other brands disappeared during American prohibition).
There's good reason for Angostura's long popularity. Angostura Bitters date back to 1824, when a German doctor by the name of Johann Gottlieb Benjamin Siegert began making them in the town of Angostura in Venezuela. They were originally intended as a restorative tonic, but became internationally popular with the rise of what we now call cocktails in the mid-19th century. Angostura bitters are now made by the House of Angostura in Trinidad and Tobago. The exact recipe is a closely kept secret, reportedly only known to five people, but the flavour is a complex blend of island spices, citrus, and bitter gentian. Angostura Orange Bitters is a far more recent flavour launched around 2007.
Angostura Aromatic Bitters is the traditional bitters used in loads of classic cocktails, including the Manhattan and the Old Fashioned. It can pretty much be used whenever a recipe calls for "bitters" without specifying a type (as can most other "aromatic" bitters).
Angostura is also terrific in a variety of non-alchohol situations from baking to mocktails to dashing on vanilla ice cream.