I've gotta say it's DS9 for me! I've been an avid Star Trek fan for as long as I can remember and as much as I loved TOS, TNG and VOY as a kid and re-watching it I'm completely engrossed by DS9, it's long-running story arks, it's huge array of recurring characters and it's build up, right from season 1 to it's brilliant finale. I have yet to watch through Enterprise which I intend to do but I sincerely doubt from what I've heard and read about it that it will even compare to DS9. And I completely agree that TOS is a must for anyone who calls themselves a Star Trek fan! It is, as has been said, the birthplace of it. Sure bad FX and some ropey acting but some truely genius storylines that are the source of everything we've come to enjoy of Trek in the years after!