TOS- Started it all, had a real sense of the good guys being pioneers in the wilderness, and produced the best Trek movies.
TNG- Great single shot stories, Patrick Stewart, really seemed futuristic at the time
DS9- Best characterization and story arcs easily. TOS and TNG may have explored strange new worlds, but DS9 really helped define them
VOY- Uh.. lost in space/TOS remix with good graphics..? I'm trying to be polite.
ENT- Season 4 showed a glimmer of what it could be, exploring a different era and telling their stories.