Okay, I'll bite:
TOS: colorful, exciting, and adult by the standards of the time. Has a certain pulpy space-opera zest and energy that (IMHO) some of the latter series tended to shy away from.
TNG: A good cast, well-utilized. TNG gets points for making better use of an ensemble cast than TOS, which (let's be honest) neglected Sulu, Chekov, and Uhura in favor of the Big Three. And Patrick Stewart succeeded in creating a different, equally impressive kind of captain.
DS9: By the end, possibly the richest, most fully fleshed out Trek series, with the best recurring characters: Garark, Dukat, etc. Despite my best efforts, however, I never found Bajoran politics or religion very compelling.
VOY: Suffers by comparison to its predecessors, but Seven of Nine and the EMH added a lot to the show. Nice production values, too. Probably the best credit sequence of any Trek series.
ENTERPRISE: Honestly, I probably need to watch it more carefully before I can make any intelligent comments on this show. (I've never written ENTERPRISE, so I haven't had cause to study it as much as the other series.)