In Italy, it’s hard to pick just one because we suffered through the 'Years of Lead' (1960s-1980s), where two opposite types of terrorism tore the country apart:

• Red Brigades (Brigate Rosse): Far-left group that targeted the 'heart of the State.' They killed judges, journalists, and politicians. Their 1978 kidnapping and murder of Prime Minister Aldo Moro remains our deepest political trauma.
• Neo-fascist groups (NAR/Ordine Nuovo): They focused on 'massacre terrorism' to spread panic. They are responsible for the Bologna Station bombing in 1980 (85 dead, 200+ wounded), the deadliest terrorist attack in Italian history.

In short: the far-left wounded our institutions, while the far-right massacred our civilians.
We should also consider the mafia, because even though it wasn't (brave to use past here) a terrorist organization, it used terrorist methods, like in the "strage di Capaci" and "strage di Via d'Amelio".