Aizen, Gin and Tousen primarily. Amagai too because he attacked Yamajii.
EDITED: There are also other Shinigami besides the ones mentioned that are considered traitors to Soul Society:
The Vizards, because they acquired Hollow powers illegally, even though they were unwilling guinea pigs in Aizen's experiments.
Kisuke and Tessai because they helped the Vizards escape.
Ran T'ao, because she helped the Bounts escape from the Shinigami who were hunting them before the main storyline (anime only).
I think that might be the complete list. If I'm incorrect, feel free to set it right.