In canon, there is only one duly appointed Grand Moff of the Galactic Empire and that was Wilhuff Tarkin. There was a self-appointed Grand Moff Randd in the Aftermath trilogy who basically promoted himself after the defeat of the second death star, but as for the Empire itself.
The system itself predates the Old Empire, with Odile Vaiken was the first to be made Grand Moff under the Sith Empire, while Wilhuff Tarkin was the first Grand Moff of the Galactic Empire, and held authority over multiple Priority Sectors.
Legends-era Grand Admirals outranked Grand Moffs, but were outranked by the Grand Vizier (Sate Pestage), the Supreme Commander and Executor (both ranks held by Darth Vader), and the Emperor himself.
Grand Admiral was one of the highest ranks in the Galactic Empire, outranking all other political and military personnel, including Moffs and Grand Moffs. Legally, no more than twelve men could hold the rank at any one time.
Grand Moff Tarkin's First Scene | Rogue One: A Star Wars Story (2016)
Did Tarkin outrank Thrawn?
He was the governor of the Outer Rim (Tarkin that is), but could he annul Thrawn's orders or did he outrank Thrawn. Tarkin probably outranked Thrawn since he was a Grand Moff (a political rank) whereas Thrawn was a Grand Admiral (military rank).
Introduced in the first film in the original Star Wars trilogy, Governor Tarkin is the Grand Moff of the Galactic Empire and commander of the Death Star.
Moff was the political rank held by the governors of each sector in the Galactic Empire. A collective body called the Council of Moffs was created by Galactic Emperor Sheev Palpatine.
Moff, also known as Sector Moff, was a political rank held by the Sector Governors in the First Galactic Empire and Imperial Remnant. It predated the Galactic Empire, having been used by both the Galactic Republic in its earliest and last days and the Sith Empire.
The enmity between the two was intensified when Ahsoka was framed for murder, and treason; Tarkin, like most of the Jedi, he refused to believe in her innocence, and the rising body count only convinced him that Ahsoka was guilty even after Padmé Amidala pointed out he had no real proof, and that the evidence that was ...
The wife of Grand Moff Wilhuff Tarkin, Lady Thalassa Tarkin was a Human female of the influential Motti family of Phelarion. Wilhuff Tarkin married Thalassa for power, not for love—he had an affair with Natasi Daala while he was still married.
At the time of The Mandalorian story, it is pretty clear that Moff Gideon is not a Sith Lord even though he has the Darksaber in his possession nor does he have any links to the Jedi. Moff Gideon is nothing more than one of the Empire's existing bad guys who officially worked for the Imperial Security Bureau.
Wilhuff Tarkin was a male human Eriaduan bureaucrat, military officer, and politician whose career spanned from the final years of the Galactic Republic into the reign of its successor state, the Galactic Empire.
It's implied that he personally dislikes and distrusts clones, probably because of their ties to the old Republic. These soldiers did, after all, maintain utmost loyalty to the Republic and the Jedi before their "programming" was activated during Order 66.
"There were always Snoke theories that he was Grand Moff Tarkin, which I always found quite amusing. You can see there are elements there. The maquette, there's elements of Peter Cushing, just the cheekbone and the profile and stuff, but he wasn't meant to be Peter Cushing. He was my Hammer reference," Manzella said.
Pryce's rank plaque is identical to Governor Tarkin's―a row of six blue tiles over a row of three red and three gold―even though he technically outranks her as a Grand Moff (or regional governor), whereas Pryce is only a planetary governor.
Affiliation(s) Ardus Kaine was a Human Grand Moff who founded the Pentastar Alignment, and was the successor of Wilhuff Tarkin as Grand Moff of Oversector Outer following Tarkin's death at the Battle of Yavin. His flagship was the Reaper.
Politically speaking, Mas Amedda is the number 2 rank in the entire Empire, just below Palpatine. Whereas Vader has no actual official rank in the Empire, politically or militarily, at all.
The Sith, Darth Vader and Darth Sidious, lead the Empire, thus they outrank everyone else. In terms of the Military Rank, Vader, Tarkin and Thrawn (Possibly the other Grand Admirals and Grand Moffs as well) were all Supreme Commanders, the highest rank. Palpatine was the only one who could override their orders.
Although Thrawn completed one of his objectives, stealing back a Sith Holocron, Zann was able to force him to retreat. This alone makes Tyber Zann one of the few beings capable of defeating Thrawn using military tactics alone.
