I like all of them, but I haven't been able to see much of Enterprise.
DS9 is probably the quinessential Star Trek. As Gabriel said, dark, and not many typical trek "happy endings".
Voyager was a great concept, the characters were perfect, but after the second season they were mishandled badly. I have a lot of ideas on how I would have scripted their journey.
For instance, I would have had them home after three or four seasons, and have to deal with the Maquis court-martials, the self-aware EMH and a recovering Borg (Seven), as well as Janeway's disregard for the Prime Directive on several occasions. That would have been intersting, I think.