1 TNG
2 DS9
3 TOS
4 ENT
5 VOY
TNG is my favourite. It was the first series that hooked me for good, so it will always be first in my heart. Plus, Picard for me is the paragon of captainhoodness.
DS9: it did anything right. If not for sentimental reasons, it would probably be my favourite.
TOS: I love it to bits, but I must admit that often it doesn't live up to its own legend. Kirk, Spock and McCoy, however, are the best characters ever.
ENT: good premise and great production values, but it played it too safe to be interesting. Characters had potential but it never came to fruition.
VOY: I really wanted to like it, but it was bland, bland, bland. Even ENT was more entertaining. It could have been at least an interesting departure from the other series, but they dropped the ball before the second episode. For shame.