Which vampire sired Angelus in the Buffyverse backstory? Darla did. She was the vampire who turned Liam, the eighteenth-century Irishman who later took the name Angelus.
Darla was originally a human woman in the Virginia Colony before the Master turned her into a vampire around 1609. As a member of the Order of Aurelius, she became one of the oldest and most influential figures in Angel's history. After siring Liam, she helped shape him into the ruthless vampire known as Angelus, and the two spent centuries traveling and killing together.
A common mix-up is Drusilla, because she later sired Spike and joined Angelus, Darla, and Spike in the infamous vampire group called the Whirlwind. Drusilla did not create Angelus; Darla did. The relationship also went beyond a simple maker-and-progeny connection: Darla and Angelus were longtime lovers and recurring antagonists in both Buffy the Vampire Slayer and Angel.
Angel eventually regained his human soul after a Romani curse, but Darla remained central to his story. She was later resurrected and became the mother of Connor, Angel's son.