TOS: Great chemistry between the Big Three and many unique and well told stories.
TNG: Kept Star Trek in the public mind which helped us get more Trek.
VOY: HUMOR, not taking itself too seriously, colorful and lively. And one of the most interesting character arcs in Trek, 7 of 9.
DS9: Politics, development of whole cultures (Trill, Ferenginar, Bajor, Cardassia..) which had only been briefly seen before. Rich cast of secondary characters.
ENT: Looks great, an arc that lasted a whole season, Jeffery Combs.