In my order of preference.
Original - brilliantly different although 3rd series patchy
DS9 - consistently brilliant
Voyager - some great moments and good story arcs
Next Gen - solid and sometimes incredibly watchable. Season 4 was shite though!
Enterprise - entertaining and well plotted. However, too much rewriting of previously established canon
2009 film good but not targeted at fans maybe.. Not as good as Wrath of Khan or Undiscovered Country. First Contact - yet more pointless rewriting of canon...