TNG season five. 'I, Borg' enough said.
"I, Borg" is from season 6, and yes that was a pretty weak season, but there are some excellent episodes in it like "Ship in a Bottle", "A Fistful of Datas", and "Chain of Command". I thought "I, Borg" was one of the better episodes of that season, actually. I think every Star Trek series got off to a shaky start, except (surprisingly), the first one. "Star Trek" had some of its most fun episodes right off the bat, like "The Naked Time" and "The Enemy Within". They were cheesy, but so very entertaining, unlike TNG and DS9 season 1 episodes which tended to be really boring and corny in a way that makes you want to cringe. I think DS9 is a little better, but they're both hard to get through. Still, worth it once you get to the later seasons, because things established in those episodes pay off in much better ones later. I've barely seen any Enterprise of Voyager, so I can't judge their seasons.