Right now Voyager is my favourite.
It has some of the best episodes in Trek, but as whole never reached it's potential IMO.
DS9 started out the weakest by far with boring Starfleet chars, but the last two seasons and wow, it turns into a completely different show. The Dominion characters were so much more interesting. Weyoun, the Founder, the Jem and their relationship with the Vorta were great. I cringe every time the story focuses on DS9 and away from the Dominion.