The Dominion War was rather broad, so I think I'll go with the Decline of Cardassia.
Honorable mentions: Data's relationship with humanity (up to his sacrifice in Nemesis) and the political prelude to Federation (which would exclude much of the Temporal War stuff).
Something I wish there were more of: the relationship between Vulcan and Romulus.
Honorable mentions: Data's relationship with humanity (up to his sacrifice in Nemesis) and the political prelude to Federation (which would exclude much of the Temporal War stuff).
Something I wish there were more of: the relationship between Vulcan and Romulus.