The First Rank was one of the various Time Lord ranks. (AUDIO: The Eleven) Retro-regeneration, the re-visiting of previous incarnations, was said to have only been attempted by Time Lords of the First Rank. (PROSE: State of Change)
Morbius, who had once been Lord President, claimed to be a "Time Lord of the First Rank" (TV: The Brain of Morbius) as did the Master. (PROSE: First Frontier)
On an alternate Gallifrey, Prydon told Romana II she was truly a "Gallifreyan of the First Rank." (AUDIO: Annihilation)