Mentioned in the movies:
Darth Plagueis
Darth Sidious (Senator/Chancellor Palpatine)
Darth Maul
Darth Tyranus (Count Dooku)
Darth Vader (Anakin Skywalker)
In the Expanded Universe (comics, novels, games, etc.), there
are dozens more, e.g. Darth Bane, Mara Jade, Asajj Ventress, Darth
Talon, Darth Sion, Darth Malak, Darth Revan, etc.