Star Trek: The Next Generation was the first Star Trek to enjoy a full, cancellation-free run. This October 2nd, its 176 episodes will be available in a new box set (watch the trailer, read the press release). Though $40/season is a fair deal, $278.89, after shipping, is still no small amount of change. And, as Trek Nation has been recently reminding us with their retro reviews, some entire seasons of TNG have not aged well.
There are many ways to separate the wheat from the chaff. Various fan collectives offer thematically-related content, so if you like time travel or Q, you're bound to be satisfied — unless you dwell on what these packages miss, such as "Future's End". For my money, The Jean-Luc Picard Collection is the best value, as I prefer episodes that offer not an anomaly of the week, but significant, focused character development. "Tapestry", "Darmok", and "The Inner Light" are worth their weight in latinum, and with the former two both coming from season five, perhaps that is the series' best season. Other fifth-season episodes "I, Borg", "The Perfect Mate" (another Picard episode, and one which first unites Patrick Stewart with Famke Janssen, prior to their X-Men team-up), Spock's return in the two-part "Unification", and the first half of "Time's Arrow" supports this theory.
But only with the new, complete DVD collection can you get all the above along with gems like "The Best of Both Worlds", "Yesterday's Enterprise", and "Relics" — so if you have money to burn, take the good with the bad and splurge on all seven seasons (plus exclusive features and documentaries). But if you don't, then what season (or fan collective) do you recommend, and for what episodes?
Also in the TNG at 20 series:
I would say that the two best seasons are 3 and 5. By season 3, the show had found its footing and had solidly grounded itself and worked out the kinks in the effects and sets.
Season 5 offered some of the best writing in the run of TNG.