Personally I think the main source of stinkers in Next Generation is Season 5. The Game, New Ground, Hero Worship, Violations, The Masterpiece Society, The Outcast, Cost of Living, Perfect Mate, Imaginary Friend. I seldom watched these during the reruns.
I agree with all of those except The Game and Masterpiece Society, but if all the seasons are on the table, are we forgetting season 1? I count at least 12 stinkers there, and that's being generous in a few cases.