I'd have to say DS9.
Just so different from all the straight lines and dull greys of Starfleet. The more organic shapes, designs and moodier lighting just helped set the scene and separate the series from everything else.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)