Star Trek: TOS - Kirk having its own interpretations of the Prime Directive
TNG - Picard sticking too much to the Prime Directive (But Picard was more of a diplomat and an explorer than a soldier)
DS9 - Sisko adapts the Prime Directive to the new realities
Voyager - Janeway ends up tossing all the rule books to the trash bin, in order to survive
Enterprise - no actual rules, as the series is the infancy of the ST Universe.

That's the simple view:)

Each series had its merits and fine moments. Yes, you might like one or another more, but in the end you watch each series for the setting:)

:) its of course a matter of taste, i was pretty bored by voyager, tng ang tos. but ds9 was ok, i enjoyed the klingon episodes. i have the impression that star trek is only successful because there are klingons. no star trek success without klingons. they should make an own series about the [url=https://en.wikipedia.org/wiki/Star_Trek:_The_Next_Generation:_Klingon_Honor_Guard]klingon honor guard[/url]!

and don't forget the klingon happy meal!