The Delta Quadrant.
Prior to the USS Voyager's accidental transport to the DQ little to nothing was known of that region of space. Indeed as Voyager traversed it towards home it had less to do with exploration (although some of it was exploratory) and more about getting home, one would argue that they did not "stop to smell the roses" as it were. As such there would be a great deal left to explore and scan.
From the point of view of the Federation, the vast majority of the galaxy has not yet been fully explored. If you were to add up Federation, Klingon, Romulan, Cardassian, and Ferengi space; it would only fill a small chunk of space along the border of the Alpha and Beta quadrants.
Here's a map:
If you say that Voyager explored most of the Delta Quadrant, and the Federation found out about most of the Gamma quadrant by the Dominion, depending on how big it is I'd say Fluidic Space. We don't actually know how large it is though.