ENT: such a refreshing take on Trek, and I love
Archer's character development from eager explorer to badass warrior. I also like the strained relationship with the Vulcans.
DS9: great show, and I really like the long story arcs. It was my definite favorite until I recently saw Enterprise. It's also quite similar in some respects to Babylon 5, my favorite sf show of them all.
TNG: after a rather slow start, I find myself very attracted by the unbridled optimism and idealism.
VOY: apart from the twist of being stranded alone and friendless in the delta quadrant, I found this show enjoyable, but not different enough from the others. Unfortunately I never could get as involved with the fates of these characters as I did DS9 or even TNG, not to mention ENT.
TOS: without it the others would never have been made. I can enjoy some aspects of the show, but the sheer sexism of it turned my stomach on far too many occasions. The show definitely shows its age, even with updated FX. That said, I've hugely enjoyed those episodes of Phase II I've seen...
Haven't seen TAS so I can't judge.