Sith Master/Legends

"Choose someone as successor and you will inevitably be succeeded. Choose someone hungrier and you will be devoured. Choose someone quicker and you won't dodge the blade at your back. Choose someone with more patience and you won't block the blade at your throat. Choose someone more devious and you'll hold the blade that kills you. Choose someone more clever and you'll never know your end. Despite these cautions, an apprentice is essential. A Master without an apprentice is a Master of nothing."

- Palpatine, as gatekeeper of the Telos Holocron

Sith Master was a title taken by a Sith if he or she had an apprentice. When an apprentice completed his or her training, the apprentice would kill the Master and inherit the title of 'Master.'

History
In the time of the Jedi Civil War, Darth Malak was sometimes referred to as a Sith Master, being the Dark Lord of the Sith and having an apprentice, Darth Bandon. Headmasters of the Sith Academy on Korriban, Jorak Uln and Uthar Wynn, held this title.

In Darth Bane's Sith Order, the Sith Master and the Sith apprentice both held the title Dark Lord of the Sith simultaneously. However, the Master was the head of the Order, and the apprentice would only become the true Dark Lord by killing his or her master.

In Darth Krayt's Sith Order, the Rule of Two was abolished, but the tradition of Master and Apprentice was still in use.

Known Sith Masters of the Sith Empire

 * Simus (Master to Naga Sadow)
 * Naga Sadow (Master to Gav Daragon and Freedon Nadd)
 * Freedon Nadd (Master to Ommin, Amanoa, Warb Null, Aleema, Satal Keto and Exar Kun)
 * Ommin (Novar), (Master to Aleema and Satal Keto )
 * Amanoa (Master to Novar)
 * Exar Kun (Master to Ulic Qel-Droma, Crado, Nayama Bindo and Oss Wilum)
 * Darth Revan (Darth Malak)
 * Darth Malak (Master to Darth Bandon and Bastila Shan)
 * Jorak Uln (Master to Uthar Wynn)
 * Uthar Wynn (Master to Yuthura Ban)
 * Darth Traya (Master to Darth Nihilus and Darth Sion)
 * Darth Nihilus (Master to Visas Marr)
 * Sith Emperor (Master to Exal Kressh)
 * Calypho (Master to Teneb Kel)
 * Darth Chratis (Master to Eldon Ax)
 * Darth Howl (Master to Eldon Ax)
 * Vindican (Master to Malgus)
 * Darth Zash (Master to Kallig)
 * Kallig (Master to Kaal, Corrin , Ashara Zavros, and Xalek)
 * Darth Rivan (Master to Darsin)

Known Sith Masters of the Brotherhood of Darkness

 * Kaan (Master to Vulta Daanat)
 * Qordis (Master to Sirak and Darth Bane)
 * Kopecz
 * Kas'im (Master to Darth Bane, Fohargh, Llokay, Sira and Yevra)
 * LaTor
 * Seviss Vaa
 * Hezzoran
 * Orilltha
 * Shenayag
 * Kaox Krul
 * Githany (Master to Darth Bane and Darovit)

Known Sith Masters in the Order of the Sith Lords

 * Darth Bane (Master to Darth Zannah and Darth Cognus )
 * Darth Zannah (Master to Hetton, Set Harth and Darth Cognus)
 * Darth Cognus (Master to Darth Millennial)
 * Darth Vectivus
 * Darth Tenebrous (Master to Darth Plagueis)
 * Darth Plagueis (Master to Darth Sidious)
 * Darth Sidious (Master to Darth Maul, Darth Tyranus and Darth Vader)
 * Darth Vader (Master to Galen Marek, Flint and Lumiya)

Sith Masters in Lumiya's Sith

 * Lumiya (Master to Flint, Carnor Jax and Darth Caedus)
 * Darth Caedus (Master to Tahiri Veila)

Sith Masters in the One Sith

 * Darth Krayt (Master to Darth Maladi)
 * Lomi Plo (Master to Morto)
 * Darth Ruyn (Master to Darth Talon)
 * Darth Talon (Master to Cade Skywalker)

Non-canon appearances

 * LEGO Star Wars: The Video Game
 * LEGO Star Wars II: The Original Trilogy
 * LEGO Star Wars: The Complete Saga